The new WordPress 3.6.1 doesn’t have this option anymore. I know there are a few code hacks out there for changing the embed size, but those affect all the embeds on the site (including embeds on regular posts).
I was wondering: Is there a way to change the embed size for bbpress posts only?
On the bbPress install package, there should be a file called bbpress.css (under bbpress/templates/default/css). Just extract that file from the package, add the code at the end of the file, and copy it to your theme’s css directory.