Yes, this has been talked over before and the conclusion was not to raise the attachment size.
YaBB actually does have a feature that would allow removing old attachments, but as stated above the best solution for everyone is to make your own web site (outstandingly easy!) or you could just put in the mission details and have a person who's interested give you his email addy. That way you'll know the beta tester is really interested and you'll most likely get feedback from him.
Long live the beta testing forum! ;D