You have now entered the process of deciding upon the scope of your game. Ask an experienced software developer where the greatest risk lies when starting a project and they will almost always talk about scope. Poorly defined scope will cause your project to drag on into development hell.
The simple advice is that until you can consider yourself an experienced developer, with a number of released games under your belt, you should make small games with the tightest scope possible. You need to iterate fast and get games out so that you can grow your experience of the entire development lifecycle. If you create and release three games in three years, you will learn three times as much as if you only release one game in that time.
The next problem is that until you have that experience under your belt it can be difficult to judge exactly how much resource you will need to make a game. It might appear that a game idea has a small scope but in practice a combination of design decisions cause a level of complexity that increases the scope in unforeseen ways. There are things to be done to mitigate this; Using an established game engine, staying within well trodden technologies and using off-the-shelf assets can all reduce the scope of your project dramatically.
I've seen a lot of game development projects fail due to a misconception about scope and resources - many of them my own. Because of this I've tried to capture the essence of the process for evaluating the scope of a game into a formula that can be applied to any game idea. This scoping tool is available for you to play around with here: www.exobyte.net/scoping-tool.html
Some things to bear in mind when playing with the tool: It's only a rough guide, the inputs are very granular and the output even more so. The goal is to give a broad idea of the resources required by a particular game rather than an exact estimation. The formula used currently comes out as being very pessimistic about the resources that will be required for very large, complex and/or high quality games. Remember that triple A game industry titles can have budgets in the hundreds of millions and over a thousand people can be involved in development.