In answer to your question, yes, mIRC has a limit. Someone else can mention what it is exactly as I can't remember.
I would assume the limit in mIRC is such that a message as long as allowed in mIRC will be displayed on most networks. Otherwise, you can enter 1000 characters in mIRC and assume it will display properly and then have only half of them show up. That's more of a pain.
However, something that might be a good Feature Suggestion, would be to have mIRC read the maximum message length from the network (I think that's possible... correct me if it isn't) and adjust its maximum message length to match that. I don't know if that would be a possibility, but it would still be a possible Feature Suggestion.
