We are planning a trip to Italy for six adults. The last time we were in Tuscany we liked the flexibility and ease of having a car. Is it realistic to think we can rent a car that will be comfortable for six and still be small enough to navigate the narrow roads and hairpin turns in the towns?
Probably not.
Streets and turns won't be a problem. Capacity will.
The largest you can get is a seven-passenger van. These things are configured such that, to get in the sixth or seventh passenger, you have to employ the back seat, clobbering the luggage space to almost nothing. The ones I've seen did not have a split back seat arrangement so that the cargo capacity could be increased by half of the rear seat. (The exception was a Peugot I had once -- probably in France -- did have the capability to fold down half of the rear seat.) Maybe you could put some baggage in the far rear and heap more in a corner of of seat, but it would still be tight.
Optionally, you can probably get two small cars for less than the cost of a van, stick the gps in the front one and use FRS radios to keep in touch. (Illegal because of the frequency usage in europe, but I've done it without being hammered.) Running two cars in tandem in cities is a mess -- I've tried it twice and won't do it again. It got so bad that we'd set an if-all-else-fails meeting point (such as the south side of the cathedral) and still spent hours walking around trying to find the other crowd.
Maybe if you packed really, really lightly?
We decided to rent two compact cars for the 6 of us while in Tuscany. And that worked really well. Even when we went to Siena we parked within eye sight of each other (Stadium Parking). Renting a van is very expensive in Italy.