The shortest novel you'd consider writing

Both the Nebula and Hugo awards put the cutoff between novella and novel at 40,000. Some more mainstream/literary awards put it even lower.

 

I tend to go by the numbers ThinkerX put up - I've seen them before (I think at SFWA).

But another useful way to think about it is the level of complexity. A short story usually goes straight from the beginning to the end. A novella will usually add a complication or another story layer, and a novel usually has several complications, and often more than one POV.

The Narnia books seem to be a aberration. I believe they are considered novels, but they all come in under 50K. Orwell's Animal Farm is very short, but I think its a novel as well.