This info is from a video on the Camtasia website. It first explains simply how to produce to YouTube, then at 1.04 describes how to produce to YouTube HD.
1. Record in at least (preferably, exactly) 1280 x 720. Is is an option under Custom. If you record in less than 1280 x 720 and follow the rest of these instructions, YouTube will put black space around the whole video and you lose precious space.
2. IMPORTANT: Then when you open the .camrec file for editing, choose the YouTube & HD editing dimensions. (If I trained you, I recommended Recording Dimensions in the past. Forget that :)
3. Do whatever editing you need to do.
4. Share to YouTube. The end. That was easy, right?
Note: when you watch the video on YouTube, it does take a few seconds for it to come into focus and be at full quality. So don't be disappointed it it doesn't look great right when you click play. Also, it will still look a little blurry at full screen on a huge monitor. Don't expect complete perfection!
If you need a bigger area than 1280 x 720 (which is unlikely), you can click the drop down menu for Custom and choose "lock to application" (toward the bottom), then choose 1280 x 720, click the lock next to the dimensions to constrain it, and then drag a corner to fill what you need to record. Producing will just make it a bit smaller.
I learned one additional cool thing from this video, which I can't believe I didn't know before: If you click "lock to application" it just resizes the recording area to the size of your window. So if I had Chrome right to the size I wanted it, instead of having to drag the edges of the green recording area, I could just click "lock to application" and it would size it. Voila. This is only helpful if you are recording one window though - if you're moving around to other windows, this doesn't work because it will just keep recording the initial window.