Here's a list of Rates & Distances plus length for a variety of train trips between Italian cities, including Rome to Florence. According to this it's 197 miles, and about an hour forty minute trip.
Train schedules and fares are subject to change
All orders are subject to a handling fee of $10.00, seat reservation fee of $11.00 and couchette reservation fee of $28.00
Reservation is mandatory on all the following trains: TGV, Eurostar, X2000, Thalys, Pendolino and Cisalpino and for all sleepers and couchettes, as well as selected EuroCity (EC), InterCity (IC) and ICE trains.
Hope my answer has been helfpul!
Try using the "build your trip" flight comparsion tool here:
Flights To Italy it will
search multiple websites and get you the best price.
Dont forget to check this links of interest:
Florence Italy Rome Transportation