the optimal way to order the correct sized shocks is to remove coils and shocks and cycle the suspension to see your collapsed length between shock mount points when the axle is stuffed. when reinstalling the springs, check extended length between shock points when the springs are installed and just about to unseat. that will give you maximum collapsed and extended lengths before your springs unseat. i doubt you'd have coil bind on collapsed length unless your shock mounts are not in the OEM locations, so that shouldnt be an issue.
hard rocker gave a reasonable method as well, but thats assuming the lift manufacturer used the correct sized shock for your jeep and lift.
the quick and easy way is to measure your coils with the tires on the ground. OEM was 12" front and 8" rear. anything over that is the lift height. you can purchase shocks based on lift height but you will generally give up a bit of travel as not every jeep is the same.
highly recommend the Rancho 5000X shocks unless funds are unlimited