When we built our pole barn I wanted to build it 13' tall. The contractor urged me to go 14'. His reasoning was that pole barn items (poles, sheet metal, etc) are all sized and pre-cut for "standard size buildings". That means 8', 10', 12', 14'. 16', etc. He told me that if I wanted a building 13' tall, that they would either have to buy 14' stock and cut it down (waste) or special order the materials (added expense and time).
If you're going to have a contractor build your carport, discuss the options with him. You may, as I did, find it's cheaper to "build bigger" rather than pay the special order fees to "build smaller"......
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS