On the youtube page, you see the "embed" code over on the right, below the "url" code.
When I copied it, it was all this:
<object width="425" height="344"><param name="movie" value="
http://www.youtube.com/v/9z7t-Ox3XvU&hl=en&fs=1"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="
http://www.youtube.com/v/9z7t-Ox3XvU&hl=en&fs=1" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>
You're going to get rid of most of that, leaving only this portion which is enlarged here:
<object width="425" height="344"><param name="movie" value="
http://www.youtube.com/v/9z7t-Ox3XvU&hl=en&fs=1"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="
http://www.youtube.com/v/9z7t-Ox3XvU&hl=en&fs=1" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>
Put another way, you only take the piece of the code in quotes, after src=
Don't take the quotes themselves.
http://www.youtube.com/v/9z7t-Ox3XvU&hl=en&fs=1In your post, hit the Flash BBC Tag (to the left of the img tag), which renders this:
[f l a s h=200,200][/f l a s h]
Change out the 200,200 to a larger dimension. (Play with it to see what you like.)
I used 500,400 on the one I edited above.
Enter the piece of the embed code which you culled out, as above.
Voila.
[f l a s h=500,400]http://www.youtube.com/v/9z7t-Ox3XvU&hl=en&fs=1[/f l a s h]
(I falsely put spaces in the "flash" word, so you could see...)