I think you are the only person ever to ask about this limit. Your original thread is Maximum number of characters in simple SIP messages (maybe a moderator can re-open and merge it?) I think most of the minority that use SIP messages use them to create mobile phone SMS messages, which have an even shorter maximum length.
RFC 3428 says that messages are generally short text messages, although it does permit other uses. It also says that out of dialogue requests with a request length (I assume that includes headers) of more than 1,300 bytes should not be issued unless the recipient and network are known to support them.