A 200 to 250 Honda is a really good one. We taught my little sister (9 at the time) how to ride a 250. It's a good size to buy because even when you get older, it is still big enough. A 600 is pretty big, but if it's a low gear 4x4 that would be alright. A 600 racing 4 wheeler will get you killed.
Talk to someone who has ridden for a long time, and learn how to ditch a 4 wheeler in case of a roll over. With ATV's that is the most important thing to learn, and always wears a good helmet. I always recommend getting something slower. Don't buy one based on power, buy it based on your ability to control it.