My husband Dave is a man of many talents, including creating 3D models of stuff using Google SketchUp (click link to download for free!). He's used it to create the model for our kitchen counter project, our hoophouse, and now the outdoor oven. This model was really helpful in visualizing what we were doing, and also helped us think about how big to make the oven.
We also wanted to be able to use the oven to bake bread, and we decided a two-loaf capacity was right for us (note that bread and pizza are not cooked at the same time).
The inside of the oven (the cooking area) is dome-shaped, so we needed to know the diameter of the smallest circle that could contain a ring of fire + a medium pizza (or a ring of hot coals + 2 bread loaves). We settled on a 45" diameter, then cut it out in cardboard and played with it for a few days...put it on the ground at the site we'd chosen, put pizza crusts on it to try it out, etc.
We were worried that it was a little too small, but the big advantage to keeping the oven as small as possible is that smaller = much better in terms of heat efficiency and consistency. As it turns out, we typically use about 2 logs* on any given pizza night (that will make about 8 medium pizzas and then, when the oven cools off a bit, two loaves of bread). We're REALLY pleased with the size we chose.
Once you know the size of that circle, you can estimate the depth and (to some extent) the height of the cooking area and surrounding mass. (The overall height of the oven is affected by the rock pillar you're about to build; you'll want the base of the cooking area to be about waist-high.)
*The logs are split into kindling, and Dave tends the fire for about 30 minutes, at which point the temp in the oven is about 700 degrees, and the mass (the dried adobe) has absorbed that heat pretty evenly. A bigger oven would require more wood and would be harder and time-consuming to come to temperature and stay there without forming hot and cold spots.