For Youtube embeds, turn off "Enable Float Alignment" under Customize>Settings>Formatting
All playlists should be embedded with "width=350," and "height=300" on both the object AND embed tag
Single video embeds are defaulted to 400x300 for now, although I think that the above "350x300" will work for those too.
These are approximate sizes, as you will have a little bit of margin on the sides, but not enough to worry about. The thing I'm not liking is Youtube's default sizing, which somehow chops off the sides of the player and in some cases shifts the player over so much that half of it has disappeared. So far this is working. I'll test this for a couple of months and see how it turns out.