I would suggest a swivel bipod. that way if you coyote hunt and have a running dog, you can turn with him, instead of picking up the whole setup. What I use and works a lot better than a harris is called a snipe pod. It attaches to the swivel, but it unfolds like tent stakes. the whole thing only weighs like 4 oz's. The nice thing about it is it not only swivels, but all you have to do to adjust elevation is move the les out, and it raises and lowers it. This is great if you have unlevel ground. check it out.
with a harris, you arestuck with the height adjustments that it comes with, so you had better like it. as far as how tall to get, it depends on whether you shoot laying down or sitting up. if you lay down, go with something like a 13" and if you sit up, go with something up to about a 28". The taller ones are nice becuase you dont have to extend them out all the way, and use them while you lay down. but make sure you get the swivel becuase if you are on unlevel ground, you will want to swivel the gun to the side so it is level. but check out the snipe pod, it is much better.