mIRC could warn users that their message -might- be cut off any time they send a message over 400 characters, however it is not possible for mIRC to know in advance whether the message will be cut off or not. The maximum message length is variable since it depends on a number of factors, such as the length of your nickname!userid@address (as seen by other users) and the target nick/channel name, both of which take away from the servers 512 characters limit per line (at least, that is what I am assuming).
I will add a "Split long outgoing messages" option to the Messages dailog in the next version and we can see how it works out. This option will split all outgoing PRIVMSG and NOTICE messages that are longer than 400 characters.
