Have you taken a beginner riding class like the MSF? Even though those are smaller bikes than you would probably buy for yourself, maybe it would give you a chance to see how comfortable you are on a bike. I hadn't ridden since I was a kid before taking the class, but since I was so comfortable on the small bike, I decided I would be ok on a brand new 805cc cruiser. I've done fine on it and haven't laid it down once, or even gotten close to. I love it, it feels like an extension of myself! Which is not to say I won't ever lay it down, I just haven't up to this point. As I said in my last post, my boyfriend has been very happy with his CBR600F4i, hasn't laid it down at all, and he was extremely comfortable in the MSF also. I know, i'm just gonna get people angry with me to say these things, but I'm just relating my own experiences (go ahead, take shots at me).

Remember I'm a noob as well, so I may not know anything!
I think the bottom line is, you need to be comfortable with the bike you choose, whether small and used or larger and new. Everybody has their own experiences and situations are different for everyone.
Good luck!