I don't even know for sure, but if we go and tweak that limit, it still won't be public. You don't need to know the exactly number of bytes a message is allowed to weigh.
Don't try to spam, and if there is a legitimate need to raise it, then raise a bug explaining why.
The forums are also not a testing ground to look for the technical limits of Xenforo and/or browsers.
We will ban abusers without option for appeal.