The thing that bugs me when buying new mohair OR alpaca cinches is some brands take the stretch into account and some don't. As a matter of fact, I have a sneaking suspicion that sometimes the same company labels them differently but I can't be sure because I have bought my cinches over several years time.
For instance, I have bought 34"s that when they arrive, brand new, actually measure 34". I have returned or sold them because I knew they would stretch out to a 36" and be too large for my horses. Then I have also bought sizes smaller than I need (say like a 32" for instance) to try to get a 34", but when the 32 arrives it measures 30", so they are accounting for the stretch. So that makes buying cinches a bit frustrating.
I usually buy Classic Equine but have also bought a Weaver. So don't know exactly who measures how, but I kind of think my Classic Equines have come both ways.....accounting for the stretch and not accounting for the stretch. But any mohair or alpaca cinch I have had stretches approx. 2 inches. They then stabilize and don't seem to stretch after that.
I think, generally speaking, you buy the size you want to end up with, just beware that if it arrives measuring the actual size you actually want, it will stretch two more inches. So you want it to arrive measuring shorter than what it says on label. They mostly arrive that way.....mostly.