I am replacing a shower/tub valve with a one handle shower valve. The new valve requires a smaller hole than what the old valve did so the new valve is sloppy in the hole. What is the best way to anchor the new valve?

A tub and shower valve should always be attached to a framing member. Usually that is a 2x4 or board between studs.

Sometimes plumbers will not do it if they feel the pipes are ridged enough to keep the valve in place. They shouldn't but they do. Nail or screw a 2x4 or board between the studs and then attach the valve with straps around the pipes. Some valves have screw holes for securing. Do not depend on the hole to keep everything in place.

I agree, the hole does not hold the valve, it should be attached to a stud. When replacing a mixer you sometimes need to open that wall opening a bit bigger and they sell escutheon plates to provide the proper trim look. How to Replace a Single-Control Shower Faucet | eHow.com (http://www.ehow.com/how_4843550_replace-singlecontrol-shower-faucet.html)