plan on the future. do you think you might get a motorhome or trailer 5 to 10 years down the road? If so go for the 16. If not the 12. Also you didn't say how high you was going with the door. I would go for a 8 ft door if you are not going to have anything high in the future. If you even think you might get a large vehicle get a 12 ft door. It is very hard to change once you get the building up.