Outlook express 6 (Attachment size)?

When someone send's me a HUGE attachment (like a wav file) It takes FOREVER to download, Im uming this is my virus program thoroughly checking out the E-mail for viruses, I discovered that you can control the size of e-mails you receive by going to outlook express: tools/message rules/mail/mail rules, you can specify what size KB you will allow?, My question is what would be a good size to allow?, I don't mind getting a few .jpegs here and there but I don't want these 15 minute download files!!!!, What would be a good KB size to use???, Hope this make's sense to ya all...????