diff --git a/const-oid/src/lib.rs b/const-oid/src/lib.rs
index 679389eec..373d92f89 100644
--- a/const-oid/src/lib.rs
+++ b/const-oid/src/lib.rs
@@ -42,10 +42,20 @@ pub use crate::{
 use crate::encoder::Encoder;
 use core::{borrow::Borrow, fmt, ops::Deref, str::FromStr};
 
-/// Default maximum size.
+/// Default maximum size: the largest BER/DER contents RFC 5280 Appendix B
+/// requires implementations to accept.
 ///
-/// Makes `ObjectIdentifier` 40-bytes total w\ 1-byte length.
-const DEFAULT_MAX_SIZE: usize = 39;
+/// Appendix B mandates support for OIDs with at most 20 arcs, every arc below
+/// 2^28, whose dotted-decimal string is at most 100 bytes. The 100-byte string
+/// is the binding limit: the leading `1.2.` packs the first two arcs into a
+/// single octet (the second arc is 0..=39 under X.660), and every further
+/// 4-octet arc costs at least 8 string bytes to spell (values in
+/// 2^21..=2^28-1 start at 7 digits), so 96 remaining bytes buy at most
+/// twelve of them: 1 + 12 * 4 = 49 contents octets, reached by twelve arcs
+/// of 9999999 below 1.2 (a 99-byte string).
+///
+/// Makes `ObjectIdentifier` 50-bytes total w\ 1-byte length.
+const DEFAULT_MAX_SIZE: usize = 49;
 
 /// Object identifier (OID).
 ///
@@ -60,7 +70,7 @@ const DEFAULT_MAX_SIZE: usize = 39;
 /// - The OID MUST have at least 3 arcs
 /// - The first arc MUST be within the range 0-2
 /// - The second arc MUST be within the range 0-39
-/// - The BER/DER encoding of the OID MUST be shorter than
+/// - The BER/DER encoding of the OID MUST be no longer than
 ///   [`ObjectIdentifier::MAX_SIZE`]
 #[derive(Clone, Copy, Eq, Hash, PartialEq, PartialOrd, Ord)]
 pub struct ObjectIdentifier<const MAX_SIZE: usize = DEFAULT_MAX_SIZE> {
diff --git a/const-oid/tests/oid.rs b/const-oid/tests/oid.rs
index 5e6005376..97754ddfc 100644
--- a/const-oid/tests/oid.rs
+++ b/const-oid/tests/oid.rs
@@ -316,3 +316,45 @@ fn starts_with() {
     assert!(!child.starts_with(oid("2.2.3")));
     assert!(!child.starts_with(oid("1.2.3.4.5.6")));
 }
+
+/// Worst case RFC 5280 Appendix B mandates support for: at most 20 arcs,
+/// every arc below 2^28, dotted-decimal string at most 100 bytes.
+/// Twelve 7-digit arcs below 1.2 spell a 99-byte string that encodes to
+/// 49 contents octets, the ceiling for any Appendix B-conformant OID.
+#[test]
+fn rfc5280_appendix_b_worst_case_fits() {
+    let oid_str = "1.2.9999999.9999999.9999999.9999999.9999999.9999999.9999999.9999999.9999999.9999999.9999999.9999999";
+    assert_eq!(oid_str.len(), 99);
+
+    let oid = ObjectIdentifier::new(oid_str).unwrap();
+    assert_eq!(oid.as_bytes().len(), 49);
+    assert_eq!(oid.to_string(), oid_str);
+
+    let round_trip = ObjectIdentifier::from_bytes(oid.as_bytes()).unwrap();
+    assert_eq!(round_trip, oid);
+}
+
+/// 100-byte dotted string from #2485 encoding to 40 contents octets.
+#[test]
+fn rfc5280_appendix_b_100_byte_40_octet_oid_fits() {
+    let oid_str = "1.3.268435455.268435455.268435455.268435455.268435455.268435455.268435455.268435455.268435455.123456";
+    assert_eq!(oid_str.len(), 100);
+
+    let oid = ObjectIdentifier::new(oid_str).unwrap();
+    assert_eq!(oid.as_bytes().len(), 40);
+    assert_eq!(oid.to_string(), oid_str);
+
+    let round_trip = ObjectIdentifier::from_bytes(oid.as_bytes()).unwrap();
+    assert_eq!(round_trip, oid);
+}
+
+/// The default capacity is what grows with this change: an explicitly
+/// smaller capacity still rejects OIDs that do not fit it.
+#[test]
+fn explicit_smaller_capacity_still_rejects() {
+    let oid_str = "1.2.9999999.9999999.9999999.9999999.9999999.9999999.9999999.9999999.9999999.9999999.9999999.9999999";
+    let oid = ObjectIdentifier::new(oid_str).unwrap();
+    let ber = oid.as_bytes();
+    assert_eq!(ber.len(), 49);
+    assert!(ObjectIdentifier::<39>::from_bytes_sized(ber).is_err());
+}