Well, I gather the thickness would simply define the resistance at rest vs. resistance while compressed. (similar to a photocell)
So, (all theoretical numbers here), a FSR with 1" thick foam might have an "at rest" resistance of 100M Ohm and a "compressed" resistance of 1MOhm, while a FSR with 1/8" thick foam might have an "at rest" resistance of 100k ohm and a compressed resistance of 1000 ohms.
This is truly groundbreaking for me. I bet I could build myself a nice little pressure pad for my Halloween display. I just need to find myself a rather large and thin sheet of that foam.
Any ideas on a conductive adhesive? They say in the instructable you find that the adhesive can interfere with the conduction between the foam and the copper board.