Hi Patrick,
Last I checked, 2 sizes. "Fine" and "Heavy". Berkshire Junctions says "Fine" is for N and HO, "Heavy" is for O and S.
I've used fine for 1/48th antenna wire, I think it looks fine (no pun intended). I think it would be ok for 1/72nd.
I recall reading that it is not really round in cross section, more flat or perhaps oval or oblong. I just looked at mine under the strongest magnification I have available at home, and if it isn't round, I can't see it. I think I've rolled it between my thumb and forefinger and seem to remember it going 'fwop fwop fwop' a little bit.
I attempted to measure it using dial calipers, got somewhere between .004" to .005". It's an elastic polymer so it might have compressed a bit between the caliper blades.
Can't comment on the heavy as I never bought any.