Gary, I have looked at both cylinders small and larger bore, externally they are both the same size. The reason for the larger bore was because the rear brakes were changed to hydraulics thus the feeling was that a larger bore for pushing more fluid was required. However as you have fitted a servo and you are changing to hydraulic rear brakes, I would be inclined to try the system as it is and then make changes to the bore size afterwards. As I mentioned before the brake master cylinder is the same size, thus you could get the cylinder bored out and fitted with a new larger plunger and seal.
Hope this info helps. Regards Leo