Typically I copy the embed code from YouTube, click the HTML button and insert it that way. If I choose to mess with the dimensions, I usually do that in the HTML view as well, but I believe you can click the video and then click the little film strip button to edit it there.
I'm fairly certain that should work for you, but if it doesn't, feel free to let me know and I'll see if I can put together a quick guide on precisely how to make it work.