Well, I'm currently reading Mark Helprin's Winter's Tale, and that's almost 800 freaking pages long.

I really don't know how things work out. I think you should write until it seems done and then decide if big pieces of it need to go.

At the same time, it's best not to keep all your eggs in one basket. Even if this book ends up not fulfilling your dream, you'll write others. You've been writing since you could hold a pencil. You aren't going to stop after one book!