Nice! The most I can get is 200 yards. I'm envious.
3/8 AR500 should hold up well at those distances. Depending on what you are hitting it with, 1/4" may do. You could also just consider these consumables and use 1/4" or 1/8" hot roll plate, then replace as necessary. A two tine pitchfork fashioned from 3/8"-1/2" rebar works well. It has enough give to absorb the energy, so ricochets are marginalized.
The tough part with small steel is adjusting for misses. The tough part with big steel is the capitol expenditure, and the weight.
I suppose the size depends on your tolerance for frustration. I would consider 2'x3' a large target, 18"x18" nominal, 10"x10" and you're an expert in my book. Consider some 4'x4' paper as well. You can always setup a small steel target in front as the aimpoint, and have some telltale to help you adjust as you train.
I'd put some windsocks (streamers or whatever) at various distances also. You mentioned a hill. Depending on topology, wind will flow in many paths. Learning to anticipate the dynamics will be easier if you can see the effects.