You want to set the size appropriate to where you are placing the resulting file. For YouTube that is now 640x480 for SD (standard def) or 1280x720 for HD. Don't bother trying to blow up the size of a standard definition image to HD.
I did find your videos, via a Google search.
I'd suggest re-exporting the video size to be larger than what you did do. If your iMovie project is set to HD, then use the larger size mentioned above, otherwise us the SD size. Those pics will look even better!
Sounds like you read my iMovie export guide. The YouTube one has specifics for YT. I may have to update them, again.
As for the 480x360 and 480x320 differences. The one on the screen shot is in error. The thing to remember when exporting to odd sizes, keep the aspect ratio the same between the width and height of the video. For YouTube, I go with their recommendations.
The background to the 480x360 size I mentioned in my guide...
I was, and still am, uploading video to a private page for family and friends. I wanted a balance between the quality of the video and file size they would have to upload. I picked 480x360 because my native material was 640x480 and I didn't like the very small quarter size of 320x240 for presenting to family and friends, so I chose the size in between. I liked my compromise because the video was still large enough to enjoy, but I could compress it more to allow for quick viewing. When I started, some people still had dial up connections to the net. Recently, I've made my videos larger.
P.S. I did a slight alteration to my web page to clarify the sizing issue.