3". i think eddie has said it before and i believe it is in the newbie section that you should get bump stops close to the hight of your lift
edit - from the newbie section
"Q: What size bump stop extensions should I install with my lift?
A: Typically, you'll want bump stop extensions that are approximately as tall as the lift you just installed on your Jeep. In other words, if you lift your jeep 2", you'll want bump stop extensions that are about 2" tall. Now, I do say 'typically' as this is just a guide to follow and not necessarily a rule. Depending on the actual height of the coil, thickness of the wire used and the amount of winds it has, the rules can change."