Do residential codes typically specify 1 1/2 inch tub and 2 inch shower drains (i.e. smaller for tub than for shower)? My plumber is telling me they are different in my town... Lafayette CA. Can't lay my hands on the local code quickly.

Never heard of that one. ALL drain lines are roughed in at 2 inches. Now the tailpieces may be 1 1/2" but the trap is 2" with a 2 X 1 1/2" reducing nylon gasket in the trap compression nut picking up the tailpiece. I don't know the code in your area but 2" drains in both tub and shower will get the inspection passed. Good luck, Tom