What is your plan with the rack? If you want to use panniers on the rear rack you need a tall rack or you`ll get heel strike. A tall rack is going to make the fold much bigger.

I would suggest a front rack, try to get hold of the Dahon front rack. You can put two full size panniers in the front, just be careful becouse they get wery close to the ground. On a folder most of the weight of the rider is on the rear wheel so luggage in front makes sense. Also makes the steering less twitchy.