There are always the exceptions to the rule, but note it is still a rule one should take note of. Sure there are people who can start on a 600cc bike and be straight but then again there are people who can barely start on a sv250 and end up riding that for some time before they feel comfortable enough to move up, and theres nothing wrong with that at all. I'm not going to sit here and preach to you like a lot of riders do but you should definitely ride numerous bikes before you buy, more goes int buying a bike than just budget and engine size. (ie. comfort, size, etc.) Me, I learned to ride on a gsxr750 and thats when i didn't know there were different engine sizes and what not. it was the only bike i felt comfortable on and it was the first bike i rode. not the first i owned though, it was my friends. even then i'm going to be buying a gsxr600 hopefully at the end of next month.

cliff notes: buy what you feel the most comfortable on, don't rush and take your time.