I just changed all the freeze plugs on my wifey's 3.8L 1998 Mustang and still have the old ones laying around. Its 1.5" outer diameter. To verify, have the parts store get one of the complete freeze plug kits for your ride & measure it.
btw, if one freeze plug is going bad chances are others aren't far behind. Your better off replacing them all in one shot then having to do it multiple times.