Also remember that when attaching binary files (i.e. and nearly any file) to a e-mail, it gets stored in MIME format which increases the actual size by 20%-30% so a 7MB File will just make it through to a 10MB limited gateway.
We also use 10MB for our internal and our client's mail systems. Seems to be a universal size.
Tyler - Parnell Geek - iPhone 3G - Lenovo X301 - Kaseya - Great Western Steak House, these are some of my favourite things.
There's no specific size, but as Exportgoldman points out, email is sent as text-only. That means binaries are converted to text, which in turn bloats the size of attachments by roughly a third.
Ten MB is probably a good compromise, depending on your storage and link capacity, but it's one of those fine lines a mail admin has to walk... it's very hard to explain to non-techie users who don't look at file sizes that they can only send a 10 or 20MB file. Also, if you set the limit too small, the messages are bounced back, since many MTAs ignore the SIZE directive in SMTP. The problem here is that users' MUAs don't tell them that "ahem, that attachment is much too big"...