My understanding is that an older specimen will be much more potent than a younger one.
That might explain why alkaloid %'s in the literature fluctuate so wildly, since people who are analyzing wild collected cactus in the Andes get nice yield, while folks in Europe analyzing young seed grown plants get low yields.
I say let em get a few feet tall, the girth thing is tricky since the various species vary wildly in their girth, bridgesii tending to be narrow while peruvianus and pachanoi seem to be fatter, if you got a terschekii or validus or something they can get 12-14 inches in diameter (!)