In reply to Phil Payne:
Have you tried rentawreck if they are still going? If you are there for a while get a taxi into the city and pickup a car as there is a big premium for hiring in airport.
Last time we went (kayaking) we booked the smallest possible car which they never have in and we were given a few upgrade to a 7 seater people carrier. This has worked a couple of times. If you do end up with a small one they are likely to gave plenty of spare big ones😀