Bruce,
We have a lot of horses we built around 31" and have since decided they are too high for anything except layout. We have started cutting about 4" off them and I like that better. I am about 6'-3". Specifically for handsawing, I would go even lower.
We also have built some ponies like Mark is talking about and have used them on every job for 4 or 5 years (they are tough). They are great for sticks that have no joinery in the middle, like joists, common rafters, etc. Otherwise you are constantly jumping the bearers to get to the middle.
