I also have used the train several times from Rome to Florence and back as well as other places in Italy. Mainly Eurostar. The suggestions from Linda and Paolo are good. I have not had a problem with a medium size suitcase. If a porter was not there to help fellow passengers did without asking. My Italian is not that good. Get a reservation, validate and be early to board. As stated there are places to stow the bag. I have not had problems with porters. If you are nervous about the space go first class with a reservation.

Enjoy the ride, Gay