probably in post.php within your theme look for:
post_author_avatar_link()
I change mine to this:
post_author_avatar_link('100')
work great for me 
one question is it possible to make different sizes for height and width ?
i want 120 for width and 160 for height
No. Gravatar(s) are square. You can force the size of the surrounding div (adding an overflow:hidden to the CSS) but nothing more.
Im not working with gravatars im working with the bravatar plugin. Is that now possible?
Why developers not add some interface parameteres and settings, I also consider the forum having too much info and too big avatar.
It would be nice to be able to reduce the amount of extra fields, info, and avatar size from just a few clicks.
The size of user avatar displayed in posts/replies is defined in /wp-content/plugins/bbpress/bbp-includes/bbp-reply-template.php.
This is the piece of code:
function bbp_get_reply_author_link( $args = '' ) {
$defaults = array (
'post_id' => 0,
'link_title' => '',
'type' => 'both',
'size' => 80,
'sep' => ' '
);
Change ’80’ (is equivalent to 80px) to a number of your choice.
While it’s not suggested that you modify core bbPress files, I know no other way (I don’t know PHP).
Hope that helps!
