http://www.heatershop.com/btu_calculator.htm
This calculator should get you in the right direction. It's the first thread link(sticky) when you open the oven building forum. Also the search function on this site works pretty well.
If I input your numbers correctly, it says you need:
good insulation/tight leakage 19,200 btu's
average insulation/average leakage 38,400 btu's
It all depends on your fabrication and building skills as to your leakage rates. I build and fabricate custom things for a living, and I had to work very hard to get slim-to-no leakage, other than the venting I designed into the oven. Start your search for a heater from there. Hope this helps