Ha! Southern California... I would say city building permits are your worst enemy! I'm just kidding... sort of... I would put the door height at 14' (that's what our shop is) because the legal limit (in many states) is 13'6, and things can happen to cause you to be over that... (air bag valve, entering at an up/down angle, jacking up the rig under the door when you have other stuff in the shop... whatever).
The 12' width door is standard, and unless you have a straight shot at the door, you still have to watch what you're doing with mirrors, etc on on a 40' conversion.
We recently put up a new building and I had the same strange fight with several building manufacturers (we have some large construction equipment that is 13') not wanting to go that high. I couldn't figure out why they were so resistant to putting up a taller structure until I learned that the structures they are erecting are not engineered for a height that would allow 14' openings. (I seem to also recall that they had to have some sort of reinforcing member that prevented it) They were basically building glorified carports. We had to go with a real steel building manufacturer and use "red iron" I beams instead of the galvanized, formed metal used in the car port type structures. This doubled the cost.
I'm don't know how much room you have, but by using the I-beams you can also get wider widths than 18' which is really nice too
I hope that helps...
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