We have a similar discussion ongoing here:
I'm starting my research on fridges. It'll go in the back of my Jeep Wrangler, eventually onto a slide in a Goose Gear system or something similar. Most of the time it'll just be me and my brother on trips. What's the most common size that everyone gets? I don't think I want to get a huge fridge...
www.overlandbound.com
There is some good food for thought in there.
It is always a balancing act between power draw/size/price/need. With the size everyone must decide for themselves, what preferences, destinations, etc. One gets along well with a 21 quarts / 20 liter box with two people, a family might prefer to have an 85 quarts / 80 liter box. Or a guide needs a hughe 105 / 100 liter plus box for a whole group of travelleres. One likes to have a large amount of chilled drinks, the next takes food for several days, which must be cooled, the next prefers to buy fresh more often. Some may also need to carry medicines in the cooler. A nice possibility is also to have two compartments, maybe one as freezer. But these boxes are simply bigger. The requirements are as varied as our vehicles themselves. This diversity makes the overlanding scene so interesting.
We use an ENGEL MT 27 F, with 21 liter or 22 quarts a little bit bigger than the MT 17
@4x4tripping mentioned above. The size is absolutely sufficient for us even for several months of travel. I have traveled extensively by motorcycle for many years without a cooler, so what we have with the rig is all luxury.