The rule of thumb is, with your foot hanging out of the stirrup, your ankle bone should hit the tread of the stirrup or slightly above. It varies a little depending on how long your legs are. Ideally when you're going over a fence, your leg be at a 110 degree angle, but you just have to play with it a bit. If you feel like you're reaching for the stirrups, shorten an hole and keep trying until you feel secure in your 2 point position.
You just have to see your distance...you don't have to like it.