In general larger is more stable, but larger also brings more expense and complexity.
Anything from 350 (Phantom size) upwards can/will be rock solid. Id advise a Phantom of you want a ready to fly, or build a basic F450 if you want to learn how to build one. Both are very stable, and the f450 has a lot of room to expand as you learn.
But, you won't get better advice than Ronan gave you... Buy a cheap micro quad like a Hubson x4, and fly it, crash it and learn to control it. It's basic controls are the same as bigger quads, and they are much more twitchy. They means you can play and learn to control it in your garage or garden, and when you master it you will have no issues with a more expensive bigger quad. And they are always great toys!
Sent by my thumb, a trusty slave to my crazy mind. The above is the ramblings of a lunatic, and should be treated as such. Terms and conditions apply.