Vent to the out site of building (naturaly) on the wall faceing inside the warm building put your filters and at least one window that way when fan or venting is on your drawing in warm air as long as vent is on there are no fumes in the other part of your shop and you can look in the window to see if paint booth is foggy or clear without having to open door (your wife or girl friend can look in too in case your on the floor because your mask failed!
) I like 5 ft. on each side of vehicle in case I need to put parts around vehicle to be painted at same time! all depends(motorcycles small booth, rigs large booth) better to have more room then not enough (dont forget you have to open doors on vehicles) dont forget to be able to have covers for filters so when you shut off vent fumes wont enter shop through filters!!
pull a big car like a 70 lincoln into your shop open doors and walk around it like your painting thats how much room you need!!
I have 2 fans you can always add another later start with one .if you only plan on painting in fair weather and dont have heat your ideas good!!