Also depending on what you plan to do with car, which you have not mentioned, you might have to find

If you have small bags - as in 22" you can fit in one car. But four adults and full size bag each plus carry-on will not fit in any european sedan. You will need something larger cheap car rentals orlando florida than that - or to have a luggage rack on the roof (unsafe cheap car rentals orlando florida for parking anywhere during the day).
We had a Fiat Bravo for four traveling for 9 days in Sicily. All our luggage was squeezed in the trunk (some of it soft-sided). It officially has a trunk of 400 liters. The Fiat Doblo (similar in design to the Renault cheap car rentals orlando florida Kangoo) has trunk space of 900+ liters.
Also depending on what you plan to do with car, which you have not mentioned, you might have to find an accommodations cheap car rentals orlando florida away from the city center or park far away from your accommodation and walk or take public cheap car rentals orlando florida transit anyway, in case you though you can avoid doing this with a car. Venice cheap car rentals orlando florida is such place.
A 22inch cheap car rentals orlando florida roller is adequate for usual itineraries. However, if you insist on having different shoes and completely different outfits each day, then you need something huge like a 26inch roller. Those who travel to Europe often find it a chore to carry more than a 22inch cheap car rentals orlando florida irrespective of the mode of transportation. One needs to leverage each item for more than one time use. Can a pair of shoes be used for more than one occasion? Do you need a medium weight jacket, a light weight jacket, a rain parka, cheap car rentals orlando florida a light sweater, and a medium weight sweater? Is light weight cheap car rentals orlando florida jacket also a rain parka and can it be combined with a sweater cheap car rentals orlando florida to serve as a medium weight jacket, etc?
I'm not sure why people find it necessary to decide for you what size of luggage you prefer. I do fine with a 22 inch roll aboard... but that's not for everyone. Take the luggage you're happy with and get a vehicle to suit. It may need to be a small minivan or people carrier though. If possible,if at least some of the party can use soft luggage it would help, as cramming non rigid bags into irregular spaces can help.
As for GPS... I highly recommend you take your own. If you don't already have one you can buy one with European maps and it may well cost less than the rental cheap car rentals orlando florida fee. TomTom is my preference but Garmins cheap car rentals orlando florida are popular. Don't buy an elcheapo off brand. You will also have the chance to learn how to use it and get the most from it. You can't do either with one rented in a strange vehicle in a strange country. I have driven thousands of KM all over Europe before GPS ... I wouldn't be without one now. They are that useful. You do still need good maps though. Pay no attention cheap car rentals orlando florida to people who have never used GPS... cheap car rentals orlando florida you wouldn't take driving cheap car rentals orlando florida lessons cheap car rentals orlando florida from someone who can't drive, would you? ;^)
" I'm not sure why people find it necessary to decide for you what size of luggage you prefer. I do fine with a 22 inch roll aboard... but that's not for everyone. Take the luggage you're happy with . . . "
Absolutely true -- however taking that sort of luggage means you must get a large vehicle. And depending on exactly where you are traveling, a large vehicle can be a royal pain. So you have to decide: Lots of luggage = large/expensive vehicle. Small luggage = smaller/less expensive vehicle. Simple as that
Second the recommendation re Tom Tom. I was having a difficult time purchasing and loading maps on my Garmin and also finding a new GPS loaded with euro maps. Finally decided to wait until I landed in Europe. Found one at the same price point at an electronic cheap car rentals orlando florida store in the Amsterdam airport. I also would take along a paper map in order to get a perspective on the total route.
They don't rent cars in Italy that are too big to drive through the streets of Italy, so don't worry about that. It isn't park anything in an urban area of Italy, so get a car your group is comfortable being in.