I've been looking into this as well. I think a 4 x 2 x 18" will work and give you enough height for a shelf too.. This is what I plan to get for my male. Constrictors NW and CSerpents also do cages, so you can check with them.

If you want the cage outside a heated room, you'll need heat tape on one end and a rhp for ambient temps. I am hoping to heat my reptile room so that rhps won't be necessary.