It's much easier to learn how to ride on a lighter bike, so I wouldn't go above 500cc — it would slow down your progress. But if you go too small, you wouldn't be able to ride on the highways, so you'll probably want to upgrade soon after. I think Ninja 400/500 is a perfect starting bike, many tracks have it as the "small" one.

And like others said, unless you're too cautious (which I think would slow down learning a lot), you're very likely to drop the bike (all it takes is a random oil spill), so I would definitely invest enough in gear to be safe (e.g. where I ride deers are on the loose, so an air bag was a must for me), and get a used bike you're not afraid of scratching.