How to change size of group thumbnails?

    if ( !defined( 'BP_AVATAR_FULL_WIDTH' ) )
        define( 'BP_AVATAR_FULL_WIDTH', 215 );

    if ( !defined( 'BP_AVATAR_FULL_HEIGHT' ) )
        define( 'BP_AVATAR_FULL_HEIGHT', 135 );

    if ( !defined( 'BP_AVATAR_ORIGINAL_MAX_WIDTH' ) )
        define( 'BP_AVATAR_ORIGINAL_MAX_WIDTH', 215 );

Also open default.css and change sizes here:

.group-box { ... width:215px; ... height:245px; ... }
a.group-box-image { width:215px; height:135px; ... } 
.single-group-box-image-container { width:215px; height:135px; ... }