There is a limit of 30MB for attachments for emails sent to your mailbox, should attachments larger than this be sent to your mailbox the sender will get a bounceback with a message similar to the following: message size (size) exceeds size limit 31457280 of server mail.(yourdomain)

Additionally, no email will be delivered that would make your mailbox exceed its quota, which by default is 400MB. You can increase the quota of your mailbox to 2GB by upgrading to a "Premium Mailbox", these are available via the "Order->Add On's" section of your customer area.