If you plan meals and shop wisely, you will be surprised by what what you can get by with. I run a 42 qt and it comfortably works for two people, for 5 days. It really just depends or how many perishable items you want to take on your trip. Weather also comes into play, with winter trips being much easier to keep items cool. On summer trips, I supplement the fridge, with the use a soft cooler or cool bag with a frozen Nalgene bottle, to keep things "cool". It works great for items that don't necessarily need to be refrigerated. Generally the items in that bag are used first and the nalgene becomes additional drinking or cooking water and the cooler bag is rolled up and stored away.
I was torn between the 54 quart and the 42 quart, and opted for the 42 as it was a better fit for the small confines of my vehicle.