dont quote me, i think rb25 turbo on an RB20 will get you about 180-190 with supporting mods, fuel pump, injectors, computer, zorst, cooler. But I think it would be stretching it to get 200. Someone will know better but thats the general power area im pretty sure.
as for the physical size of the turbo It's slightly larger.
hope this helps
edit: http://www.project32.net/img/turbo_actuators.jpg - 25 turbo on left, 20 turbo on right