You are too big for the KX 100.
Your going to want a KX 125. I was in your position at one point, and I am only 5'7 115 pounds and I ride my KX 125 fine. I love it, and I am very happy I went with it over the KX 100.
Were you going to buy the KX 100 new? I was going to buy an 04' KX 100 for 3250$, when the dealer showed me some leftover 2003 KX 125s for only 3200$!!! I thought it over and went with the 125 and I don't regret a second of it. Expecially at that height your gonna be cramped on a KX 100. Go with the 125, see if any dealers have any leftover models. I ended up paying only around 3800$ with all the tax + extra fees + shipping. Saved over 2200$ when compared to a new one!