Good beginner dirtbike?

Like Kevin said, you don't buy a full sized dirt bike based on your height. 125's do tend to be taller than a 450F, but they are all within an inch or so of being the same height. So here's the deal, forget about how tall you are, that makes absolutely no difference whatsoever. Obviously at 6'3" you need a full sized bike which means 125/250 2 stroke or 250/450 4 stroke. First off, no 2 stroke race bike is a good beginners bike, there are in fact horrible beginners bikes. A 450F is only for very experienced racers or someone over 200 pounds. Since you are neither of those things your choice is quite simple...a 250F. You should not even consider a 450F in your wildest dreams. I own a RM125, CRF250R and CRF450R and ride a YZ250 on a very regular basis so I am very familiar with the power characteristics of all 4.