Florence isn't that big a city, only about 350000 I think, and it wouldn't take long to ride out of town into the Tuscan countryside, which is sensational. Plus, Florence is pretty special if you're interested in art. If you did want somewhere a little smaller, then Siena is spectacular and not far away. Arezzo (easy driving distance of both Florence and Siena, for sightseeing) is another lovely smallish Tuscan town.
Milan is a genuinely big city. Worth visiting but I wouldn't make it a base for a cycling holiday. Venice may be the most beautiful city on earth, but you'd want to be staying outside it on dry land if cycling was your priority.
In France, Avignon would be a good base. There are lots and lots of possibilities, though. You might want to get somewhere actually in the French Alps or Pyrenees if mountain passes are what you want.