I was asked to send a lot of large images to someone, via e-mail (I know), and set about breaking them apart into separate messages.
I found out quickly that anything over 10 MB would generate an error due to message size, so I broke them apart into several smaller messages. Some were 7 megs (total size including headers and such), and some were 9.9 megs. All under 10, so I figured they'd be OK.
Nope. Instead of complaining about the sizes, it said to try sending them later. Tried again, and that gave the same error. I made them 7 megs by moving some images to new messages and they all went through just fine.
So, I don't know what the sweet spot is, but I figure 7 MB is safe.
And no, it's not an issue with headers since the numbers I quoted above are including the headers.