High Speed Trains

You can put a sizable piece of luggage in the rack above you. If the car is not crowded and the seats near you empty (you should be able to see reservations) then you may be able to put it near you. Otherwise, at the end of the cars there are luggage racks, sightline will depend on what seats are reserved, but generally I have not been concerned about security. Making a periodic check is sufficient with stops being the only real concern. A fast train from Florence to Rome may not have any other stops anyway.

If you are concerned about security of your luggage, in case it's so big you have to leave it out of sight in the racks on the extremities of the car, buy a cable lock and lock them to something. There are no intermediate stops between Rome and Florence however, therefore it's unlikely someone is going to take your suitcase and jump off the train at 200 mph.

Small to medium sized luggage will fit either overhead, or in the spaces between the seats. With the big ticket items, there are racks situated about 1/4 way down the coaches. Whether you will be able to see them depends on your seat assignment. If no one is sitting next to/across from you, then you may be able to situate it closer to you. However, I highly recommend that you leave that "large piece of luggage" at home and try and downsize to a smaller piece of luggage. Not sure how much traveling you will be doing, but that sized luggage gets really annoying really fast.
Otherwise, just bring something like a bungee cord/buckle thing and just simply secure the luggage to the rack if you have to leave it on the separate racks. No big fancy lock job needed, but if you simply make it 10-seconds more difficult for someone to grab your stuff, they'll most likely move on to something easier to grab. At the stop, just peek over and keep an eye out until the train starts moving again.