i'm not trying to claim that I'm an expert, I don't even own a streetbike. But like the guy who was asking for advice, I've ridden dirtbikes for about 4 years now (not as regularly lately, I sold mine). I weigh 270, and my main bike was a yz250. I've also ride (now) my friends' cr250r, crf250x, cr500, xr400r, ktm520, ktm 320, and ktm125. My yz250 was plenty of power, even for a guy my size. I could lift the front tire all the way into 4th gear with only some power reeds and a bill's pipe. The bike had no problem supporting my weight (after a new DGY racing suspension).

One of my buddies bought a new yamaha 600 streetbike about 2 years ago, and has let me ride it a couple of times. His bike has plenty of power for me, and considering I've ridden some of the more powerful dirtbikes made, hopefully this will help out for comparison. There's no need for anything more than 600cc (imo from mostly dirtbiking experience) for a beginner streetbike