Are you sure thats not an ISP issue? Do you have an attachment limit size with your ISP?
I only ask because if you've unticked the box there shoudn't be a problem. You could always try clicking the "disable plugin" option as well, and see if that helps. The AVG icon in the taskbar will go two shades of grey to indicate that its not working "fully", but you can change it back afterwards.
What about the Resident Shield? Have you tried turning off the "scan email files" option in the "Properties" Tab? I cant think for a moment that would have any effect, but stranger things have happened...
I am using Thunderbird and i am now aware of yousendit!
It does seem a strange one - as their does not seem to be any adjustable feature in AVG that lets you specify the size of files that it will send/receive. There is such a feature in Thunderbird - but this was not exceeded in this case.
In any case - turning off/exiting AVG before sending the email did work on this ocassion.