When (How?) Does a Novel Become a Novel?

I began a new novel recently and, while I’ve been contemplating the idea for a while, I’ve had trouble feeling the project is gaining traction. I am questioning whether the idea has sufficient coherence and gravitas, along with the legs for me to complete it. A novelist-friend kids me about being too dogged in exploring ideas that might best be abandoned. I tend to wrangle an idea to the bitter end, insisting on…