Ah! Hit 20,000 words today. That seems like a milestone for me. The point at which I know this is more than just an idea--it's actually going to become a book. 20,000 words is the no-turning-back-now point. If the idea was going to fizzle, it would have done so already. All of my books have been this way.
And all of the books I have abandoned have been before reaching that length.
Some people can know off an outline . . . or even just as the idea percolates within their mind. But I have to get into the meat of the book before I know for certain that it can hold out for the long run.
Well, I guess that's not completely true. I have the second book of my fantasy series all planned out and have written a very long synopsis for it, actually. But I know that one can fill a book.:) Maybe it depends on the book. The second book in my fantasy series I am very familiar with already because I've written, edited, revised, and polished the first one. I guess it's like having the trilogy a third of the way finished already. But the YA I'm working on right now is its own world with completely new characters. Maybe that's why I wasn't sure at first that the idea was big enough.
Sometimes it's the opposite; you have an idea that's just too big to fit in one novel. I guess that's how a lot of series are born.
How about you? When do you know your idea is big enough for a novel?