From Lunarpages Web Hosting Wiki
What is the maximum size of an e-mail attachment?
For Linux servers (Basic, Business, Reseller, and Starter) there is a 10mb restriction on sending and receiving attachments per email message. The total size of each email our customers can receive including headers, the body, and attachments is limited to 50 Megs.
If you have attachments larger than 10 megs, please upload them to a subfolder of public_html on your account instead and create an FTP account in FTP Manager to provide access only to that subfolder for other people or you may upload to a password protected subfolder and provide the login and URLs to the other person(s).
Your account can store as many email addresses and emails as your plan's limits will allow.
You may also need to increase the Quota limit on your email address to prevent the mailbox becoming full. The default email account quota is 10 megs. You can change this limit by going to your cPanel and clicking on Mail, then Manage/Add/Remove Accounts. Click on Change Quota next to the email address to be changed. Please set to an actual high enough number and not "unlimited" which tends to cause issues.