Exporting from iMovie '09 to YouTube - Video Length?

I've been exporting HD videos from iMovie '09 directly to YouTube using the 720P option from the Share -> YouTube menu. However, it seems I get a vague error message when I upload a video that approaches 5 minutes in length.

1) Can anyone confirm a video length limit for HD videos on YouTube? I know for SD it is 10 minutes but could not find a separate limit for HD video.

2) I understand YouTube limits uploads to 100MB but my video files are several GB before iMovie converts them to "YouTube HD" format. The problem I'm running into is I don't know when I approach the 100MB limit since the formatting is all done automatically through iMovie/YouTube.

Is there a way to figure out the file size of my YouTube HD movie before iMovie tries for half an hour to upload it and get rejected?

More info:

Right now I have it figured out that if I do the automatic export from iMovie to YouTube HD, that YouTube will accept it if it is somewhere between 4-5 minutes.

I don't know the exact length that it allows and it'd be mighty time consuming to figure it out since iMovie first converts my HD project and then tries to upload as much as possible before finally (half an hour later) getting rejected.

I'm also not sure if the movie is rejected on length alone or on file size; if it's the latter, then the length allowed may vary depending on the movie, right?