Guess I would be asking you how you use the horse? Is this for ring riding, trail riding, games?
I never went for the mohair, since it could attract moths to damage it in storage and it is hard to wash well or often. It can shrink when washed. So I use fuzzy string girths of nylon that requires no special care, lets me wash it often to be clean, prevent any rubs from sweat or dirt. Hang to dry always. They are not roper wide, but are double thick which seem to prevent pinching too. I do always pull the front legs forward to get any wrinkles out from under the girth area, before riding.
I fit mine so the rings end above the elbows, as other said. My horses do things that make them sweaty. Could be ring work, trail riding or practices of various speed classes. I don’t care for the roper width because it holds heat in, less skin and hair exposed to the air for cooling. Horses here get sweaty often, so being able to self-cool is important with hours of use. Mine all wear breast collars every ride. So even when using the old horse with low withers, I didn’t need to girth her really tight to prevent slipping.
I don’t rope off mine, so they don’t need the spread out width of those roper girths to absorb shock of an animal hitting the rope. If you ride a number of horses with one saddle, you might consider neoprene girths, which can be easily disinfected between animals to prevent spread of fungus or other problems. They also are rather grippy, so saddles are less likely to slip under you. Horse does sweat under the neoprene, which holds in heat. They come shaped, but not as wide as roper girths, if you like shaped girths.
