In response to your question, the maximum standard usually allowed is 5mb per email. File sizes larger than that can slow the server response, should regular transfer of greater than 5mb be allowed.
A simple solution for this is to place an automated ftp upload page on your website where the end user can visit, choose upload, browse for file and click ok. This way they do not need to learn the ins and outs of ftp protocol.
Another option is to encourage the use of file compression [zip, tar, rar] prior to transmission of the file.
Hoope this helps.