My carport is 14' wide and is totally taken up by the main slide being out. If it was 12', I could not begin to put the slide out. As it is, the slide comes within 3" of the wall (slide is about 33" deep).
I actually started out with just a 12' wide slab and quickly realized it would be tight once the carport was errected. You can see a seam in the concrete where I had it widened 2' more and I'm really glad I did.
If you have room, go at least 14' wide if not more. Remember, it may not cost anything more than the concrete - if you use it. If you just use gravel it may not cost any more at all.
The height is 13' at the outside corners. That was really too tall. It doesn't look like it in the pic but the AC clears it by a good 20" or more. I can walk around on the roof, bent over but fairly comfortable.