FunfDreisig's procedure at the start of this thread tells you, e.g., 1 1/4 inch socket or 32 mm. I splurged and bought a set of large metric sockets at Sears to get the 32 mm. Probably unnecessary, since 1 1/4 inch = 31.75 mm.

If you do as I do and take off the engine cover instead of the intake tube, you also need a long extension for the socket wrench. If you take off the intake tube, you may not need the extension.