Sounds like you are making a charcoal grill and not a smoker. If so, remember a grill is hot and fast so excess oxygen is needed to keep the coals burning. I would first think that your intakes are too small for that but your question was the exhaust. If it is a grill, exhaust is irreverent but should be, in my opinion, at least as big as the intake so as to not cause a restriction.
I may be making too many assumptions....might need a bit more info.