Recommended Mailbox Maximum Size and Email Attachment Policies

Does anyone have some advice for me as to what I should set as the maximum mailbox size for Exchange 2000? I had one mailbox recently for a manager weigh in at 2.5GB. I have about 40 email users and I want to keep the nightly backups at a manageable size. Is there a mailbox size that starts to negatively affect perfomance of that users account?

Also, what would you recommend I set as my email attachment policy? Currently, I'm blocking .scr, .pif, .vbs, .eml, .cpl, .com, and .chm. I'm thinking I'd like to add .exe to that block list. Zip files frequently have viruses but I know my users get those from time to time and I don't want to inconvenience them or discourage them from asking for compressed attachments. I'm using Vamsoft's ORF SPAM filter to drop emails containing the aforementioned attachments. My options with that program are to drop the email or let the email through and replace the attachment with a text file explaining that the attachment is not allowed. Any advice? Tips?

Thanks.