Luggage Trenitalia

There are luggage racks, usually at the back of the car or at either end, if your suitcase is too large to fit on the overhead rack. As long as you can carry your luggage, size is not a problem. You need to be able to carry you luggage quickly when boarding or exiting the train.

Every train station we encountered in Italy had stairs to navigate. We were glad that we had carryon size luggage.

Didnt think about that, so at the station you have to use stairs.

Not all stations, but many. If they are pull through stations (with no "end" and the trains pull in from one side and out the other) they will have stairs. Some may have elevators, but they may not be operational and also may be slow or have a line. Roma Termini and Napoli Centrale are terminus stations, so it is more likely you won't have to use an elevator for that, but if you are doing any other stations nearby, there will be stairs. At least 2-3 on the trains to get on/off. Lugging large luggage anywhere is not fun for me, I don't know your tolerance. Are you doing a taxi to/from those stations? If you were taking any further or prior public transit there will probably be stairs. After only my second day of train travel on my last trip (first, arrival day, second, my first location change) I wanted to chuck my carry on size piece of luggage off the train it was annoying me so much hahaha

Roma Termini and Napoli Centrale are flat, no stairs unless you're coming into Naples on the commuter train from Sorrento. Most other stations we've been to have stairs.

Yes, take cases that are as light as possible if you are taking a few train trips.
Some of the older trains have 2-3 steep steps up into the train from the platform, so you need to be able to lift luggage UP to get on board.
The other thing is, if your case is big and so are a lot of other people’s, there might not be much room to store it in the racks at the end of each car.
You can’t store them near your seat in aisles either, no room.
Have a look at
https://www.seat61.com/

Lots of info and photos there of what to expect.

You will have to lift the bag when you enter and leave the car (a couple of steps).
Generally speaking, it is wise to use a smaller bag for train travel than for air travel. After all, in air travel you mostly have to bring your bag from the airport door to the check-in area and from the pickup belt to the exit door, really very short stretches. In a large railway station you may have to walk longer stretches, lift the bag, enter/exit the car, get to the track trough an underway passage (stairs down and up). Bring a smaller bag and save yourself trouble.

In the future, limits to baggage size are expected on trains as too many people bring too large bags, but they are not enforced now.

There's no size limits HOWEVER, as others have pointed-out...you'd do yourself a big favor by not lugging around a larger than carry-on size. At the end of each car is a luggage area, they can fill-up so, be aware, there's also overhead racking above the seats, most model cars can fit a carry-on sized suitcase up-above. Worst case scenario, one of your party has to stand near the doors minding the luggage that doesn't fit anywhere.

There will be people with 25' suitcases and roller-duffels, the train is the big transportation method not just for tourists but for Italians themselves, either getting to work, visiting relatives or, moving so, you'll see all sorts of bags and carry-on's. Seeing people struggling, dragging, and hauling large sized luggage will make you quickly reconsider what and how you pack.