Lots of good info here. I would also suggest going with a 12 Ft side wall. I am not sure what type of construction you are concidering, but I would suggest steel. It is cheaper, and goes up much quicker. Also with steel you can go with a 1 pitch roof and keep the overall hieght about the same as your house.
I built mine 30x40x12. I used steel construction and alumnafoil insulation. Very easy to put up and has a R19 insulation factor. I have two projects, lots of junk, an room for my extended cab driver. It is tight, but the room is good. I still wish I had 10 more feet on the length. You cant have too much room.