A good rule of thumb is 1.5 sq. feet of make up air per stack dryer. Yes i know its less with some mfg'rs but its better to have it and not need it then need it and not have it! I upsize every install for possible future needs (both opl and coin). All newer dryers seem to be more efficient in terms of both electricity and natural gas use. I am still waiting for the side by side comparison between dryer mfg'rs that shows time to dry, ACTUAL btu's used and KW's used. I've seen a number of skewed and or bias comparisons.