I smoke in a 16 x 14 shed with a few BOTLs from time to time (more as it gets colder). I can't see the bathroom fans pulling the smoke out. I have in each shed window a window fan unit, equivilent to 2 heavy duty bathroom fans in each window. Even with the tall ceiling they aren't quite sufficient. I will look up their cfm's in the book when I get home and post if it's there.
As my shed is heated with 2 space heaters, I understand the desire not to loose heat. It is a no win situation, your gonna lose heat to vent the smoke. You just need more heat than ventilation. I am going for a bigger circuit to the shed, a few 1500w heaters, and a better fan installed in the roof to pull the smoke out. Trying it on the "cheaper" route has just not done it for me.
Also need to bury a cable out to the shed for the mini tv
edit:
Woot!!!!!!!!!!! Post 2000 for me. I think I earned a smoke when I get home