I mean a mass that is appropriate to the use of the oven. If you plan on cooking 2 pizzas, use the steel. If you plan on 10, you need 2-3 inches of refractory material. If it is to be used for continuous, or occasional party use, you need 4-5 inches of mass. By discrete, I mean isolated from the enclosure, and it needs to be insulated by a material other than the thermal mass itself. Insulating brick do not work for thermal mass, steel does not work for thermal mass, sand is not an insulator, nor is glass, cobb, or tinfoil.
Shape is really preferential, a barrel oven requires more finesse, but you never put the fire in the back, it runs down the side. A round oven radiates evenly and gives you the most square inches of usable floor space, and it needs to be large enough to allow you to move the pizza around to make for an even bake.
I have a barrel, 22"x36", and I use 4" of coals and fire on the left hand side, leaving me a very narrow area in which to cook. I have to constantly turn the pizza or it will burn at Neapolitan temps. I do not normally cook Neapolitan, so it is not an issue, but if that is what you want, you need to go round, low dome and as large as you can afford to heat.
There are other ratios and dimensions that are important, but until you wrap your head around the futility of a steel wood fired pizza oven, they do not matter.