Size of exhaust in lid

I I've seen a lot of talk aboutt the air inlet ports, size, construction, etc., but not a lot about the exhaust. I'm helping a friend build a UDS and so far, we've used a 55 GAL drum and have 4 air inlet pipes, 24 in high. 2 are 1" and the other two are 3/4" dia pipe. Is a 2" dia hole big enough for the exhaust, with this amount of air coming in, or do we go bigger, and would a chimney, 3-5" high help the draw?

Thanks for any help you can give us.

Martin