Beverages would also factor in fridge size. If the wife enjoys white wine, husband likes beers and cold Gatorade, kids like orange juice and the baby drinks milk... these drink can easily consume 1/3 of the fridge. If the family drinks minimal refrigerated drinks, then a smaller fridge will do fine.
Some foods like bread, bagels and tomatoes which you could normally leave out at home would get annihilated in a parked, hot vehicle.... but you aren't going to fit it in your fridge. I've had good luck putting those in an insulated soft cooler or something sort of insulated and I'm using reflective window shades for the trunk area. Also helps with fridge power consumption.
Lastly, it is nice to have a little extra space... not only as a buffer but also the fridge performs better internally with some air space. We don't often pack food the for the entire trip but rather restock along the way. Sometimes the food product is not available in small quantities.