There is no limit on the size of the e-mails that you send or receive other than the maximum disk space for the account. It depends on how much space is available in the receiving mailbox, in order for the recipient to be able to get the mail message.
Please note that the HTTP protocol itself (which is used for attachment uploads in the Webmail) is not too suitable for big file transfers, and timeouts may occur.
If you wish to send large attachments, you better use a standalone mail application, such as Outlook or Thunderbird - please check our Configuring an e-mail program (Outlook, Thunderbird, Mail, etc) article for instructions.
There is no maximum size limit for sending messages via our SMTP servers; however, large messages may not be delivered successfully if your Internet Service Provider does not allow the distribution of large messages, or if the mail server of the message recipient is configured to reject incoming messages above specific size. If you encounter such a problem, you can upload to your hosting account/website any files that were attached to the message, and provide a link for direct access to the files on your hosting account/website. This should reduce the message size significantly, and result in a successful message delivery.