I have a slight wobble in the harmonic balancer and am looking at replacing it. Is the Powerbond OEM unit that Mace stock the standard diameter as I don't want to underdrive?

All of the Powerbond Standard and Powerbond Street balancers have essentially the same overall dimensions as the genuine balancers they replace, unless clearly stated. Therefore, in your case, the Powerbond Standard and Powerbond Street balancers will be the same diameter as the factory balancer they replace.