I'm a huge George Eliot fan and I just finished her novel, Romola, set in Florence in 1492, possibly her least popular novel. Some critics say it's her best novel, and she always swore it was her best. I guess she should know. It's quite a history lesson, too. I would find it easier to put the story into historical context if I knew the population of Florence around that time. The novel makes is sound like a small town, by modern standards.
We can't be too precise because it's not easy to find accurate records for specific years that far back.
But we can say that around 1492 the population of Florence was around 40-50,000 inhabitants.
50,000 is the number quoted in Italian Urban Population 1300-1861 under the column 1500, while the number for the fifteenth century is reported as 37,000. Those two numbers give us a pretty good idea of the size of Florence at the time, definitely not a small town, but not a big city either.
Sadly the author of that database detailed his methods in an article he published, which is not freely available online.
You should know, though, that a couple of centuries before the date you specified Florence was a much bigger city. We have a precise number for a specific year, 1339. Giovanni Villani established the population at 90,000. He drew this estimation from "the amount of bread consistently needed by the city", which sounds ingenious. This number is often quoted in Italian sources.
The lower number in the fifteenth and sixteenth century is due to the Black Death which hit in 1348.
In 1351, right after the plague, we have another number: 45-50,000, extrapolated from the number of houses. The plague almost halved the population of Florence, and in 1492 it still had not returned to its previous levels.
Source for the rest: Treccani encyclopaedia.
Edit: for immediate clarity
I believe it had a population of 80,000 in 1300; during the High Medieval Period (900ish-1300ish) of economic prosperity and population growth. But that was before the Great Famine of 1315, and even worse to come, the Bubonic Plague of 1347. Over a century later, the city was still less than half it's population prior.
I'm reading Machiavelli:A Biography by Miles J. Unger, so I only have an ignant estimate; but around 1500 the population was probably still less than 40,000.