Smaller bikes are better to start off on. You won't find yourself in a world of trouble with no exit in sight if you make a wrong move on one... like you would if you went with something larger.
Not to mention the fact that, being 17, your insurance cost will be a major part of your decision. a 250R should be reasonable, whereas most ins companies wont even write policies for larger bikes.
My advice is to shoot lower. a 600CC supersport is not even close to a good decision for a new rider.
By the way... welcome to the board!
If you want to save time rather than wait for an answer, there is a search function here, as well. Being a new rider, there is a 99% chance that any questions you want answers to have already been asked before, and you will find some very sound advice among the other threads started by new riders in the past.
Finally, do not forget to budget for riding gear. Helmet, pants, coat, gloves, boots. Very important stuff.
I used to race dirtbikes, and I have been riding since I was 3 as well. Dirt is a good place to start... you know what is what and where the controls are. The rules change drastically when you move to pavement, though. If you go slow, the transition will be a smooth one. Just don't take a bigger bite than you can swallow.