RFC 1991 only gives them 4 bytes (32bit); not sure if there have been any later additions to rectify this but I don’t think so since even the latest RFC (9580) has them listed as 4 bytes…
https://datatracker.ietf.org/doc/html/rfc1991
https://www.rfc-editor.org/rfc/rfc9580#section-3.5