Exhaust fan help needed

I have a shed, aprox 25' by 8', with an unfinished beam roof. There are gaps between the roof and the walls to provide some airflow, but they are nothing significant. Later on, I would like to wall off the inside into two smaller rooms, to break things up a little, but air supply is the most important concern.

What kind of / how many fans do I need to be able to stay inside while brewing? Bonus: the shed has two doors that open out from the middle and are quite tall, roughly like so ____\ /____ I would prefer to leave at least one of these closed, if possible.

CO monitors have already been purchased, and I have an electrician ready if I need to run out extra current to power the fans.

Thanks.

Go electric... I guess you are using gas... I say put a fan in a window and open one door. This will be too much but too much is just right in this case.

I would try to calculate it by using some reference materials like http://www.electricsuppliesonline.com/howtosiexfa.html

Go to a local electrical, heating supply company and see what they can tell you. No point in doing the work and finding out it's not sufficient.