GeekDesk provides the dimensions of the desks, the large may not be able to accommodate your existing track, as the desk hardware mounts across the underside of the desk near the back. My 'small' frame takes up about 10" from the back of the desk, so I just got a 14" humanscale track. (My desk is only 25" wide)
This is slightly imperfect, as my tray can not fully retract under the desk, but the benefit of freeing the work surface and enabling more adjustment outweighs this concern.
My only major concern is whether my Humanscale keyboard tray can fit underneath—it's about 21.5" long and 5.5" wide.