Why would driverless cars be smaller? They still need to be able to carry the same number of passengers; the only difference there is that one of those passengers is no longer also a driver.
If you're deploying a fleet of robotaxis, most of your fleet can be vehicles that only carry one or two passengers at a time on short trips. If a family wants a ride to the airport, then they'll specify that they need a larger vehicle for more passengers and luggage, etc. So you can buy a fleet of mostly smaller cars with a shorter range, and compliment them with a few minivans for long range trips. Overall your fleet will be quite efficient.