Also, please instruct me on the correct way to ask this question.
Hi Joan, Loren wrote about that in "How to insert a video" in "How to do it!"
When you've used the "Media" button and you're pasting in the embed code from YouTube or wherever, it'll have "height" and "width" numbers. You can change those numbers, either before hitting OK, or later on in HTML mode.
Presumably you want the new height to keep the same proportions (aspect ratio) the video originally had. You can calculate it from the original height and width, and the new width.
In your example, the original dimensions were 640 px wide x 360 px high. If the new width is 500, the new height should be 360*500/640 = 281 and a hair. (The ratio of the widths, 500/640, is how much the height needs to be multiplied by.)
This might help some brains: if you write it out with labels, the right ones will cancel out to show that the calculation is correct:
360 old height * 500 new width / (640 old width) = 281.25 new height