Change the avatar ratio in bbPress login widget

You can filter 'get_avatar':

add_filter( 'get_avatar', 'wpse_67657_new_avatar', 10, 5 );

function wpse_67657_new_avatar( $avatar, $id_or_email, $size, $default, $alt )
{
    // create a new img element or …
    $new = str_replace( 's=40', 's=80', $avatar );
    $new = str_replace( 'avatar-40', 'avatar-80', $new );
    $new = str_replace( " height="40" width="40"", " height="80" width="80"", $new );
    return $new;
}

Filtering the size only is currently not possible.