I made mine about 12" wide and about 18" tall for standard birds.
There are Silkies in some of the coops and bantam Cochins in another and I often just don't raise the door up real high for them so the wind or rain doesn't come in but with the standard Cochins I have to raise it all the way up for them.
I think mine is only 12 inches high (10 inches wide), and my girls (breeds listed in my signature) have no problems going out or in. If I had a rooster, I might want it about 2 inches taller??? If you go taller, and think it's too much, you can always lower the door a bit. There's really no "wrong/right" answer.