To see if a really big limit breaks anything, you could set it to a million, say, and then use the lorem ipsum text generator to create the text for you. You can then keep doubling that number until it chokes. Must try this myself someday. Imagine having to read through a wall of text a million characters long lol.
I recommend doing this on a test install on your local machine only, for obvious reasons.
Requirements: XenForo 2.2.0+ Description: Allows to set maximum message length trough permissions. PS: "Maximum message length" and "Maximum profile message length" options are not ignored and must be higher than or equals to the maximum...
xenforo.com
I'm using this mod right now and it works as advertised.
You can use it to give admins (or members with paid upgrades) larger limits than regular members.