I've raced motocross and also raced woods events and also do a lot of other off road riding on quads and UTVs. As the guy before me said, CC size isn't always going to directly reflect power. A crf450L has a 450 race bike engine and has much much more power than say a DRZ 400. I would suggest going with a CRF 300L. It will have enough power to handle almost all trails outside of steep steep inclines. It's a great bike and doesn't require much maintenance and is relatively inexpensive. Also couldn't go wrong with the DRZ 400 but I think it might be a little heavier and slightly less capable. When riding off road, too much power is often a hindrance rather than help. You can often go faster in the woods on a 250 race bike compared to a 450 because it is lighter and you are not having to fight the power output of the bike as much. A 250 4-stroke or 125 2-stroke bike would be a good option for starting as well, but they aren't typically set up for registering for road riding.

Good luck if you decide to get a bike. Take it slow at first, off road riding is much harder than road riding and there are a lot of lessons to learn that are best not learned the hard way. ATGATT