Many years ago I became fascinated with earth oven construction after reading a book called, “The Bread Ovens of Quebec” by Boily and Blanchette. This is not an easy book to find, but if you come upon it, then buy the book. Just recently, however, the book became available for a free download on the web. Not only is it filled with interesting folklore, but also there’s real information on clay oven construction. The authors found that the most successful ovens had door-opening dimensions that related to interior oven height. Kiko Denzer and others have used this formula and I saw no need to reinvent something that has been working for a very long time. Therefore, I use it too, but with some wiggle room. Opening height should be 63% of the height of your sand mold. If your mold is 14" tall, then the height of the opening should be 8.82".
0.63 x 14 = 8.82 Generally, the opening width is half the diameter of the floor. If your floor diameter is 22 1/2 inches. Half of this is 11 1/4 inches.
I am emphasizing the word "generally" because they may be problems with this rigid formula. I believe that the French Canadians were much more interested in baking bread than they were interested in making pizza, and with such a narrow opening it might prove difficult to fit a peel inside or maneuver a pizza around.
I have spoken to some people whose one regret concerning their ovens is an opening that's too narrow.
So, if you're making pizzas, then be sure to make the opening wide enough to accommodate them. Yes, you'll lose more heat with a larger oven opening, but that's better than having an opening so small that your oven won't work properly for you.
Wide enough for pizzas