I'm sure you know how the sizing works. But I'll explain for anyone who doesn't know.
20x10x9 = 20" overall height x 10" tread width x 9" rim diameter.
For rear tires, you can put whatever height you want, as long as it fits under the fenders. Width needs to be 0"-2" larger than the rim diameter, so the tread doesn't balloon up. I like 10" wide tires on 10" wide rims, so the tread sits completely flat, to get more contact area. The rim size has to be the same.
Front tires, you want ballooned, if you're offroad riding. You probably have 5" width rims. Go with a 7" wide tire, to purposely balloon the tires.
Choosing your tire height, depends on what you're doing with your bike. If you'r riding MX, you want a smaller height tire, to get the bike a little lower, and to help the engine to be able to use all of it's torque. Trail, you'd want something taller, to make more clearance under the bike.
Now, what do you plan on doing with your bike?