bbPress.org

You will have to deal with that padding anyway.

To change the avatar size, create a new theme by copying the bbpress-twentyten theme to your WordPress themes directory and changing its name in style.css and then at line 53 of the file bbpress/loop-forums.php (can be the wrong file) look for bbp_author_link function. You will see the size as 14 there, change it to a bigger size.