I have a 2004 Suzuki Eiger 400 4x4. Still runs and works well, but if I was to look for one now I'd want a Honda or Yamaha. I only bought the Suzuki because a friend was selling it to me.
OP, what state are you in? Do you get snow? If you want to ride much in over say 5" of snow you're going to want 4x4. I don't know how familiar you are with quads, but a 4x2 will go some amazing places with good tires, but get into much snow and you'll want the 4x4. It's obviously going to help in the mud too, but I've been through some pretty good mud with 4x2.
As far as size, my first quad was a 250. It was fine, but slightly under-powered. A lot of guys will throw out big numbers, but I'm more than happy with the power of my 400. And factor in sometimes you want to be able to get off and manhandle the quad in places. The bigger the engine, the heavier, and harder it is to do that.