Is there kindof like a "rule of thumb" out their for how much diskspace/bandwidth to provide per email?

As far as I know there isn't any rule of thumb...generally though email size is up to you how big you want to create the size of the mailbox. You are normally limited though by the size of your hosting account. So if you are on a 1 GB hosting plan for example, you could make your email be up to 1 GB depends on what you set it at your control panel.

Most budget/shared hosting accounts are not equipped to store large amounts of email. For those types of hosting plans, its best to use Outlook or some other sort of mail client that downloads email to your computer.

Hi splitz,

It really depends on the size of the plans you are offering. I don't think you would want to give less than 20MB, especially if your users are going to be using IMAP which stores mail on the server. Try to encourage POP3 usage when you can.

Really depends on if you want to store e-mails on the server or you tend to send e-mails with large attachments then you need to allocate more space to the account.
If you don't send large attachments and don't store e-mail on the server then 20-30mb is fine :)