It's not so much the diameter as it is the thermal mass. 5" difference in diameter won't matter if your 42" has a thick dome with a heavy layer of cladding. The question of how large do you go, is subjective to personal preference.
There are a number of people that successfully use small diameter to make Neapolitan pizza....Omid for example, has a 25". It comes down to how you want to manage the oven...it is more difficult to manage pizza making, the closer your pies are the heat source.
Also, having built and used an oven based on the Pompeii plans, I can assure you, it is a more than capable oven design for pizza....and, the hemispherical shape is more novice friendly to build than a traditional styled Neapolitan oven.