diff --git a/imap-codec/Cargo.toml b/imap-codec/Cargo.toml
index aef25190..e9b7f06d 100644
--- a/imap-codec/Cargo.toml
+++ b/imap-codec/Cargo.toml
@@ -78,6 +78,7 @@ ext_login_referrals = ["imap-types/ext_login_referrals"]
 ext_mailbox_referrals = ["imap-types/ext_mailbox_referrals"]
 ext_metadata = ["imap-types/ext_metadata"]
 ext_namespace = ["imap-types/ext_namespace"]
+ext_status_size = ["imap-types/ext_status_size"]
 ext_utf8 = ["imap-types/ext_utf8"]
 # </Forward to imap-types>
 
diff --git a/imap-codec/fuzz/Cargo.toml b/imap-codec/fuzz/Cargo.toml
index f82d1719..a2881d49 100644
--- a/imap-codec/fuzz/Cargo.toml
+++ b/imap-codec/fuzz/Cargo.toml
@@ -25,6 +25,7 @@ ext_login_referrals = ["imap-codec/ext_login_referrals"]
 ext_mailbox_referrals = ["imap-codec/ext_mailbox_referrals"]
 ext_metadata = ["imap-codec/ext_metadata"]
 ext_namespace = ["imap-codec/ext_namespace"]
+ext_status_size = ["imap-codec/ext_status_size"]
 ext_utf8 = ["imap-codec/ext_utf8"]
 
 # IMAP quirks
@@ -41,6 +42,7 @@ ext = [
     #"ext_mailbox_referrals",
     "ext_metadata",
     "ext_namespace",
+    "ext_status_size",
     "ext_utf8",
 ]
 # Enable `Debug`-printing during parsing. This is useful to analyze crashes.
diff --git a/imap-codec/src/codec/encode.rs b/imap-codec/src/codec/encode.rs
index edbaddb5..9e57c914 100644
--- a/imap-codec/src/codec/encode.rs
+++ b/imap-codec/src/codec/encode.rs
@@ -859,6 +859,8 @@ impl EncodeIntoContext for StatusDataItemName {
             Self::Unseen => ctx.write_all(b"UNSEEN"),
             Self::Deleted => ctx.write_all(b"DELETED"),
             Self::DeletedStorage => ctx.write_all(b"DELETED-STORAGE"),
+            #[cfg(feature = "ext_status_size")]
+            Self::Size => ctx.write_all(b"SIZE"),
             #[cfg(feature = "ext_condstore_qresync")]
             Self::HighestModSeq => ctx.write_all(b"HIGHESTMODSEQ"),
         }
@@ -1700,6 +1702,11 @@ impl EncodeIntoContext for StatusDataItem {
                 ctx.write_all(b"DELETED-STORAGE ")?;
                 count.encode_ctx(ctx)
             }
+            #[cfg(feature = "ext_status_size")]
+            Self::Size(size) => {
+                ctx.write_all(b"SIZE ")?;
+                size.encode_ctx(ctx)
+            }
             #[cfg(feature = "ext_condstore_qresync")]
             Self::HighestModSeq(value) => {
                 ctx.write_all(b"HIGHESTMODSEQ ")?;
diff --git a/imap-codec/src/response.rs b/imap-codec/src/response.rs
index 0dd5e8a1..898a15e7 100644
--- a/imap-codec/src/response.rs
+++ b/imap-codec/src/response.rs
@@ -510,6 +510,14 @@ mod tests {
                 b"".as_ref(),
                 Response::Data(Data::Capability(Vec1::from(Capability::Imap4Rev1))),
             ),
+            #[cfg(feature = "ext_status_size")]
+            (
+                b"* CAPABILITY IMAP4REV1 STATUS=SIZE\r\n".as_ref(),
+                b"".as_ref(),
+                Response::Data(
+                    Data::capability(vec![Capability::Imap4Rev1, Capability::StatusSize]).unwrap(),
+                ),
+            ),
             (
                 b"* LIST (\\Noselect) \"/\" bbb\r\n",
                 b"",
diff --git a/imap-codec/src/status.rs b/imap-codec/src/status.rs
index 75a659e7..70ba448c 100644
--- a/imap-codec/src/status.rs
+++ b/imap-codec/src/status.rs
@@ -32,6 +32,8 @@ pub(crate) fn status_att(input: &[u8]) -> IMAPResult<&[u8], StatusDataItemName>
             tag_no_case(b"DELETED-STORAGE"),
         ),
         value(StatusDataItemName::Deleted, tag_no_case(b"DELETED")),
+        #[cfg(feature = "ext_status_size")]
+        value(StatusDataItemName::Size, tag_no_case(b"SIZE")),
         #[cfg(feature = "ext_condstore_qresync")]
         value(
             StatusDataItemName::HighestModSeq,
@@ -87,6 +89,11 @@ fn status_att_val(input: &[u8]) -> IMAPResult<&[u8], StatusDataItem> {
             preceded(tag_no_case(b"DELETED "), number),
             StatusDataItem::Deleted,
         ),
+        #[cfg(feature = "ext_status_size")]
+        map(
+            preceded(tag_no_case(b"SIZE "), number64),
+            StatusDataItem::Size,
+        ),
         #[cfg(feature = "ext_condstore_qresync")]
         map(
             preceded(tag_no_case(b"HIGHESTMODSEQ "), mod_sequence_valzer),
@@ -99,8 +106,51 @@ fn status_att_val(input: &[u8]) -> IMAPResult<&[u8], StatusDataItem> {
 mod tests {
     use std::num::NonZeroU32;
 
+    #[cfg(feature = "ext_status_size")]
+    use imap_types::{
+        command::CommandBody,
+        mailbox::Mailbox,
+        response::{Data, Response},
+    };
+
     use super::*;
     use crate::testing::known_answer_test_encode;
+    #[cfg(feature = "ext_status_size")]
+    use crate::testing::{kat_inverse_command, kat_inverse_response};
+
+    #[cfg(feature = "ext_status_size")]
+    #[test]
+    fn test_kat_inverse_status_size() {
+        kat_inverse_command(&[(
+            b"A01 STATUS frop (MESSAGES SIZE UIDNEXT)\r\n",
+            b"",
+            CommandBody::status(
+                "frop",
+                vec![
+                    StatusDataItemName::Messages,
+                    StatusDataItemName::Size,
+                    StatusDataItemName::UidNext,
+                ],
+            )
+            .unwrap()
+            .tag("A01")
+            .unwrap(),
+        )]);
+
+        kat_inverse_response(&[(
+            b"* STATUS frop (MESSAGES 8 SIZE 44421 UIDNEXT 242344)\r\n",
+            b"",
+            Response::Data(Data::Status {
+                mailbox: Mailbox::try_from("frop").unwrap(),
+                items: vec![
+                    StatusDataItem::Messages(8),
+                    StatusDataItem::Size(44421),
+                    StatusDataItem::UidNext(NonZeroU32::new(242344).unwrap()),
+                ]
+                .into(),
+            }),
+        )]);
+    }
 
     #[test]
     fn test_encode_status_data_item_name() {
@@ -112,6 +162,8 @@ mod tests {
             (StatusDataItemName::Unseen, b"UNSEEN"),
             (StatusDataItemName::Deleted, b"DELETED"),
             (StatusDataItemName::DeletedStorage, b"DELETED-STORAGE"),
+            #[cfg(feature = "ext_status_size")]
+            (StatusDataItemName::Size, b"SIZE"),
         ];
 
         for test in tests {
@@ -138,6 +190,8 @@ mod tests {
                 StatusDataItem::DeletedStorage(u64::MAX),
                 b"DELETED-STORAGE 18446744073709551615",
             ),
+            #[cfg(feature = "ext_status_size")]
+            (StatusDataItem::Size(u64::MAX), b"SIZE 18446744073709551615"),
         ];
 
         for test in tests {
diff --git a/imap-types/Cargo.toml b/imap-types/Cargo.toml
index 1c3136ba..c8ce2c3c 100644
--- a/imap-types/Cargo.toml
+++ b/imap-types/Cargo.toml
@@ -27,6 +27,7 @@ ext_login_referrals = []
 ext_mailbox_referrals = []
 ext_metadata = []
 ext_namespace = []
+ext_status_size = []
 ext_utf8 = []
 
 [dependencies]
diff --git a/imap-types/fuzz/Cargo.toml b/imap-types/fuzz/Cargo.toml
index 1c79fedd..20e0a67d 100644
--- a/imap-types/fuzz/Cargo.toml
+++ b/imap-types/fuzz/Cargo.toml
@@ -21,6 +21,7 @@ ext_login_referrals = ["imap-types/ext_login_referrals"]
 ext_mailbox_referrals = ["imap-types/ext_mailbox_referrals"]
 ext_metadata = ["imap-types/ext_metadata"]
 ext_namespace = ["imap-types/ext_namespace"]
+ext_status_size = ["imap-types/ext_status_size"]
 ext_utf8 = ["imap-types/ext_utf8"]
 # </Forward to imap-types>
 
@@ -33,6 +34,7 @@ ext = [
     #"ext_mailbox_referrals",
     "ext_metadata",
     "ext_namespace",
+    "ext_status_size",
     "ext_utf8",
 ]
 # Enable `Debug`-printing during parsing. This is useful to analyze crashes.
diff --git a/imap-types/src/response.rs b/imap-types/src/response.rs
index 3fb43582..c71ce498 100644
--- a/imap-types/src/response.rs
+++ b/imap-types/src/response.rs
@@ -1106,6 +1106,9 @@ pub enum Capability<'a> {
     /// NAMESPACE extension (RFC 2342)
     #[cfg(feature = "ext_namespace")]
     Namespace,
+    /// STATUS=SIZE extension (RFC 8438)
+    #[cfg(feature = "ext_status_size")]
+    StatusSize,
     #[cfg(feature = "ext_utf8")]
     Utf8(Utf8Kind),
     /// Other/Unknown
@@ -1153,6 +1156,8 @@ impl Display for Capability<'_> {
             Self::QResync => write!(f, "QRESYNC"),
             #[cfg(feature = "ext_namespace")]
             Self::Namespace => write!(f, "NAMESPACE"),
+            #[cfg(feature = "ext_status_size")]
+            Self::StatusSize => write!(f, "STATUS=SIZE"),
             #[cfg(feature = "ext_utf8")]
             Self::Utf8(kind) => write!(f, "UTF8={kind}"),
             Self::Other(other) => write!(f, "{}", other.0),
@@ -1223,6 +1228,8 @@ impl<'a> From<Atom<'a>> for Capability<'a> {
             "condstore" => Self::CondStore,
             #[cfg(feature = "ext_condstore_qresync")]
             "qresync" => Self::QResync,
+            #[cfg(feature = "ext_status_size")]
+            "status=size" => Self::StatusSize,
             "uidplus" => Self::UidPlus,
             #[cfg(feature = "ext_utf8")]
             "utf8=accept" => Self::Utf8(Utf8Kind::Accept),
@@ -1321,6 +1328,16 @@ mod tests {
         assert_eq!(Capability::ListExtended.to_string(), "LIST-EXTENDED");
     }
 
+    #[cfg(feature = "ext_status_size")]
+    #[test]
+    fn test_capability_status_size() {
+        assert_eq!(
+            Capability::from(Atom::try_from("status=size").unwrap()),
+            Capability::StatusSize,
+        );
+        assert_eq!(Capability::StatusSize.to_string(), "STATUS=SIZE");
+    }
+
     #[test]
     fn test_conversion_continue_failing() {
         let tests = [
diff --git a/imap-types/src/status.rs b/imap-types/src/status.rs
index 0408bc94..5af6064b 100644
--- a/imap-types/src/status.rs
+++ b/imap-types/src/status.rs
@@ -33,6 +33,10 @@ pub enum StatusDataItemName {
     /// The amount of storage space that can be reclaimed by performing EXPUNGE on the mailbox.
     DeletedStorage,
 
+    #[cfg(feature = "ext_status_size")]
+    /// The total size of the mailbox in octets.
+    Size,
+
     #[cfg(feature = "ext_condstore_qresync")]
     HighestModSeq,
 }
@@ -67,6 +71,11 @@ pub enum StatusDataItem {
     /// The amount of storage space that can be reclaimed by performing EXPUNGE on the mailbox.
     DeletedStorage(u64),
 
+    #[cfg(feature = "ext_status_size")]
+    /// The total size of the mailbox in octets. Refer to RFC 8438 and RFC 9051,
+    /// Section 6.3.11 for more information.
+    Size(u64),
+
     #[cfg(feature = "ext_condstore_qresync")]
     /// The highest mod-sequence value of all messages in the mailbox.
     /// This is the same value that is returned by the server in the HIGHESTMODSEQ response code in
diff --git a/justfile b/justfile
index 1054f849..b2f6a282 100644
--- a/justfile
+++ b/justfile
@@ -65,6 +65,7 @@ cargo_hack mode: install_cargo_hack
         ext_mailbox_referrals,\
         ext_metadata,\
         ext_namespace,\
+        ext_status_size,\
         ext_utf8 \
         --group-features \
         quirk_crlf_relaxed,\
@@ -96,6 +97,7 @@ cargo_hack mode: install_cargo_hack
         ext_mailbox_referrals,\
         ext_metadata,\
         ext_namespace,\
+        ext_status_size,\
         ext_utf8\
         {{ mode }}