See my other post in the related Feature Suggestions thread. There's no effective way of mIRC deciding what it 'should' and 'should not' send. Besides, a file limit makes the incorrect assumption that large files will be illegal and small ones will not. About 75% of all JPG's you'll find being sent around IRC or any other protocol are either copyrighted material or porn (even legal porn being sent on IRC is mostly illegal in the sense that there's no verification of the receivers age).
