Good question!
I spent lots of internet time trying to figure that one out.
I was SO FRUSTRATED that no one listed height of different breeds, just weights.
I found that for "large chickens" the size recommended is what you found, 14"tall and 12ish wide.
I had my DH make the door 18" high since I stress about those things, and 12" wide.
My brahmas are still little though....
But that door, with my little brahmas going in and out, looks HUGE!!!!
I am quite sure that the rooster could duck his head if he had too. But looking at my monster size door, I am sure your door will be perfect.
Do you already have your Brahmas? What color did you get?
I got 11 light Brahmas, but they are very far off standard. (as in the have black everywhere, not just neck and tail).