I NJ you need 2" for a shower. The theory is that the curb will hold less water than a tub, so you need to drain faster. However, I have seen 1.5" shower drains that work just fine. If your work is not gonig to be inspected, I would stick with the 1.5". If it is going to be inspected, you can try bouncing it off your inspector first and see what he says before breaking the slab. You never know.