Remember that it's okay to start small and work your way up. There is no shame in starting with 150 or 230 and progressively upgrading to a 250, 350, or 450 four-stroke or a 250 or 300 two-stroke. Don't cave to peer pressure or think you have to prove something to your buddies by buying a big bore bike. We've talked to a lot of 450 guys who didn't know what they were getting themselves into and wanted to downsize to a 250. A 450 will beat you up and take you for a wild ride if you're not ready for it. Our 250s have never let us down, and we've taken them over some nasty stuff.
At the end of the day, it comes down to you and your preferences. Do a lot of research. Get expert opinions. Attend demo rides. Compare different bikes. Your local dealership can be an excellent source of information. Become informed.
Check out his video from ShawTVSSM for more tips on selecting your next dirt bike.