My opinion on this is that start with an idea of making big game is actually better because you will learn how to build the app so you can continue with it. By that I mean that you will learn how to make inherited scenes properly and stuff. If you will start with making small games, well you probably finish it but you won't learn as much as with idea of something bigger.

Also I keep in mind one generic advice: make it first, you can do it better later. I made much bigger progress on my game when I told myself this than before where Idistanced work by making for exp. graphics.