You are working to balance two conflicting requirements with your oven opening.
First, the smaller the opening, the better your oven is at holding heat. If the size of the opening relative to the volume of the oven is too large, it will be difficult to keep your oven at high heat, and it won't hold heat for baking. There is a U.S. made commercial oven that has this problem, and I have heard numerous restaurant owners and pizza chefs complain about it. I had a small (26") oven at an Italian house, and the opening (16" x 10") was too large (relative to the oven volume), and it just couldn't hold heat.
Second, the opening needs to be big enough to easily move food in and out of the oven. If the opening is too small, it can be a pain just using it. I have that problem with my Scott oven.
For the opening width, 18"-19" is pretty standard. It gives you room to work (and as Robert says) see the fire, without over doing it. You can go a little wider, but I wouldn't go past 20". That width will let you get just about any pizza in and out of the oven.
Opening height does depend on dome height, and 60% is an OK rule of thumb -- though it is something of an urban legend. There is no perfect formula, and I have never seen it used in the context of an Italian brick oven. For example, the ancient ovens have really high domes (you can almost stand up in them), so the opening height to dome height ratio on those is tiny. Equally, as brick ovens gets larger and the dome gets higher, there is no reason to make the opening taller than 12"-13" -- the size you need to get a roast in and out. So in that case the ratio of opening height to dome height gets smaller and the ovens simply perform better.
If you have a 42" oven, with a 20" dome, an 19"W x 12"H opening should work nicely. Good access, good thermal characteristics.
If you have a 36" oven with a 17"-18" dome, an 18"W x 11"H opening would be good.
If you are building a Volta Bassa (low Naples style dome), you should bring your opening down an 1" or so -- but makes sure you can still fit in a turkey.
If you want to maximize heat retention, you can always arch your oven opening (higher in the middle for a roast), but lower on the sides for heat retention.
If you are really into maximum design, you can have a local metal fabricator make you a cast iron door opening like this: http://fornobravo.com/commercial_piz...istorante.html
Wide at the bottom, high in the middle and closed in at the sides. Now that is cool.