Hired a car from Sixt Chambery - on picking it up the boot looked too small for our bags and smaller that the Peugeot 3008 that was given as a typical car for the class.
However, the staff tried everything to help us, including taking our bags to the car and looking at other options. Ultimately they didn't have a different car but with their help we managed to squeeze everything into the BMW 225.
The car turned out to be great, as we found out after that the rear seats slide forward to give a bigger boot. Would have helped a lot if the staff knew this, but they were very new - both just a few days in, so not their fault. Overall a very positive experience.