I have a question about the size of rental cars in Italy. We’ll need a car for 4 days for 4 adults. Our luggage will be 4 carryon suitcases and assorted smaller bags. What size vehicle do we need? I’m not sure how big the cars listed actually are, and I don’t really believe their info on how many suitcases they hold.

For example, I know in the US a Toyota Corolla is considered a midsized car by the rental companies and a Camry is full sized, but I know nothing about the cars listed for the different classes in Italy so it’s hard for me to picture how much they hold.

Any advice?

You should research all the types of cars being considered. We rented a Corolla staionwagon in France last summer and it was fine for three of us and should fit you with 4 carryons as long as your other bags are not too large. Can also put small bags in between the two in the back if necessary

Obviously you appreciate that rental cars are given as or similar?
The Toyota Corolla is officially a compact car (C segment in Europe). This could mean restricted leg room in the rear.That depends on the passengers height. Had a VW t-roc recently. Rear leg room was almost nonexistent. This is officially classed as a Subcompact crossover SUV.
Classes of cars are confusing at times.
https://en.wikipedia.org/wiki/Euro_Car_Segment

Four people plus bags the safe things is a station wagon , van or SUV.

A normal five seater car isn't likely to have enough trunk space.

Some of the cars might be for sale in the US. BMW 3 series. VW Golf. Drop by a dealer and ask to look behind the seats.