From what I know, there's no specific limit, just what's within the realms of sensibility.
For reference, my Rush vids are, with one exception, in the 7-9MB range, which I got the impression was considered fine seeing how I could only film them in a very awkward manner (screencapped vids will be smaller). Otherwise they would probably have been thought rather large. My Dead Line vid is 14MB, which is a bit big, but I hadn't quite figured out the camera I was using at that time.