Originally Posted by FBinNY

The balance issue is definitely not connected to the size issue. There may be an alignment problem with the frame, or even somethingm as simple as a loose headset, or it might just be you.

If there's a mechanical problem, I'd expect the shop to correct it, but I'd be surprised if they took it back after you've ridden it a while simply because it's too big for you. It's possible and something you should take up with them, especially if they fitted you in the store and suggested this size, but if you picked the size on your own you may be out of luck as far as returning it.