The same idea could be a novel or a short story. It depends on how much you flesh it out with character depth, subplots, or making the main plot a bit more convoluted. Same as what JCornelius said, really.
If your novel looks like it's going to end around 50,000 words but you were aiming for 70,000 words, then adding in any of the above will make it longer. Obviously you have to do this carefully, to maintain continuity and also a coherent plot structure. Some writers prefer to outline first to get all this stuff right from the beginning. Others (like me) will plan the story while writing it, and go back and make major edits if need be. And then there are writers who will mix and match these methods. Whatever works for you.
It's misleading to focus on whether ideas are good enough. It's what you do with the ideas and how you write it that makes it a novel or short story (or anything else in between).
If your novel looks like it's going to end around 50,000 words but you were aiming for 70,000 words, then adding in any of the above will make it longer. Obviously you have to do this carefully, to maintain continuity and also a coherent plot structure. Some writers prefer to outline first to get all this stuff right from the beginning. Others (like me) will plan the story while writing it, and go back and make major edits if need be. And then there are writers who will mix and match these methods. Whatever works for you.
It's misleading to focus on whether ideas are good enough. It's what you do with the ideas and how you write it that makes it a novel or short story (or anything else in between).