I'm a backpacker with a small kelty bag.

Do yourself a favor and buy a small one. I've seen people on their first days on RTW trips with huge backpacks and they're already complaining. It all depends what kinda backpacking, if you're camping more than larger is needed but if you're hostel/hoteling it, then I would opt small. A good collaspable one is what I have.

you'll thank yourself when you're wandering around some dusty town in India, Vietnam or China looking for one of the two cheap places to crash in town.

Another tip, since you'll probably gather up souviners, bring stuff you won't miss, so if you brought a shirt and buy a souviner one, toss an old one and just wear the new one. I do this all the time.

I did meet one guy who showed up in BKK with just the clothes on his back and a small backpack with necessities and bought everything there... He was also complaining about how heavy his bag was when I had met him. (met him in china)