For our shared/sdx/reseller hosting, and users with cpanel on their vps/dedicated, the maximum email size is 50MB. This is the default set by the developers of mail server and they say it is safe value. Anything higher than that can be abused for denial of service.
Note: 50MB does not mean you can attach exactly 50MB file. The file attachment is encoded for email transmission and can take more space. For example, a 10MB file may take 12MB in email after encoded data.
If you need to send/receive large files, please use FTP.