Site subscribers can send up to 20Mb per-post, free accounts have a limit of 5Mb.
The size check on our server tries to compensate for the fact that files get a variable amount larger as they are re-encoded for sending by email (your email software does this automatically), but the occasional message that appears to be just about the right size before sending may get rejected due to this uncertainty. We apologise for any inconvenience this may cause.
You will receive an error message back to the address you sent from if your message is too big.