Assuming you're trying to minimize voltage drop, #10's have a resistance of about 1 ohm per 1000'. At 200' out and 200' back, you'd be looking at 4 ohms total. With a 10 amp load (a water tank heater possibly?) you'd lose 4 volts. Well within reason.
I'd use 3/4" PVC, and 4-#10's, as suggested above. Direct-bury will almost always get nicked or eaten through by various varmits, PVC is a bit tougher. Plus, if you ever need to add more stuff later, (no one ever does, right? lol) with PVC it's usually doable, with direct-bury, it's a little harder.