Well you see the problem here is that mIRC can never know the max message length of the current irc server, because it is never sent in any raws. Otherwise, it would be easily scriptable and you wouldnt have the problem im thinking you are.
Keep in mind, mIRC can send more than double the standard irc message length, so I don't see an issue unless you're specifically trying to prune your own input to the server
Generally though, something around 400 would be the best bet.