There is no "standard height" for an HDT. Each will vary by body style, tire size and so on. If your plan is to put just the truck in the garage ,12 feet would be more than enough. Assuming of course you have adjusted the stacks accordingly. As an example my Kenworth W900 is 10' 1" to the top of the air horns. It has 1100 24.5 tires. If it had low pro 22.5 tires it would be several inches lower.I suspect most trucks will easily clear a 12' door. Of course if it was possible to go to 14' you could get the trailer in as well.