IDK about girth not affecting potency, what if you have two cuttings, one two times the girth of the other? The bigger one definately will be more potent, just due to its bigger surface area/mass. More green flesh just under the skin, the most potent part.
And as far as fresh grams, i couldn't say. I do know about 12" of 3" diameter bridgesii dehydrates to ~30g dry material. And thats a good dose for me. But its fresh weight is heavily dependent on how much water it got before it was cut from the plant. IF it was dehydrated, it will weigh a LOT less than more hydrated cuttings. Water is heavy.
The general consensus for a good dose of bridgesii, is 8-12" length of a 2-4" diameter cutting.
I've extracted that to ~250-300mg of mescaline hcl, tannish stuff after an acetone wash. I'd say thats a good solid medium dose, at least for someone small like me.