I think you are drawing strong conclusions using as a start things which are wrong, things Fiore clearly didn´t say or simply didn´t show much importance.
As it has been said already, North Italian people is still today much taller than the average south italian inmigrant commonly seen in the american continent. I would think that it was something much more evident in the past. Particularly between the high classes who used the swords we can associate with Fiore. Which left us without a clear evidence of such longer vs shorter blades since, as far as I know, there is a lot of variation of swords with the same general style.
Fiore makes reference to the convenience of using one posta or another according to sword lenght (Ex: porta di ferro), which is a clear hint the Fiore was pretty flexible to that point.
Sword lenghts are a matter referred in later manuals, like Vadi, and they follow the, common also in spanish and portuguese manuals, that the two handed should reach the armpit with the pommel. But this measure leaves out most swords we can associate with Fiore´s times and places.
Also there are clear differences between Fiore and the later masters like Vadi which lead to think about a longer sword in the last ones (like the footwork, but this is my opinion and off topic).
But your main argument, the expression "spada a doy mani", simply is not enough. Fiore says indistinctly "spada a una mano" "spada a doy mani" "spada d´una mano" and "Spada de doy mani". The two latter could certainlñy mean a different sword. But not necessarily a longer blade. You are just guessing that a sword for two hands has a longer blade. It could be stouter, heavier, longer hilted, etc. But I don´t realy care because the use of "a doy mani" and "de doy mani" is erratic in Fiore. And "spada a doy mani" means "sword at two hands". The confusion exists only because in romance languages both expressions "a una mano" and "d´una mano" are not so different. In english you have (and I suspect you think) "sword at two hands" and "two-handed swords" which are different and clear things. Ok, we don´t have that. We say "a" and "de" indistinctly in many ocassions. Just like Fiore. And it can mean that the thing is different as well as just the way is affected. So, you simply cannot draw a general fixed rule about that.
Fiore is very precise with those things he considered important and sword lenght, or even sword type, is not one of them. Techniques are described as useful with "good sword" or "not important if it is not very long" or " useful also for pollaxe, heavy stick, light stick" etc. The guy was very straightforward and not shy to repeat himself in every aspect he considered important so I´ll consider that what he said is simply what he said and no hidden key secret is there waiting to be discovered.
There is, however, a place in Fiore where it seems Fiore is actually describing a technique not possible with "spada de doy mani", the attack to the knee. But these kind of things I would take as more probe that Fiore´s system is one generally indifferent to sword length (between a historical range) and thus needed of clarifications where sword lenght is relevant.
Adn what it is also important: Romance languages, not having aglutinative resource, develops different words for different things. Fiore never says "spadone" for example. These kind of words (spadone in italian, montante in spanish) appear later in the XV cent. Vadi´s times.
My conclusion is that there is nothing there to see. It is simply what it seems: For Fiore there is not a fixed length of blade and, due to the remaining swords of the period, this lenght is shorter than in latter manuals.
Edit: There is a nice example of this: "Tiro del giavellotto a due mani" which simply implies both hands are used and not an special type of javelin. And there are many examples of the use of "a due mani" which actually implies a different object too.