What size cooler do I need for a weekend with four people?

Get a 65-quart cooler. Anything smaller, like a 48-quart, is too tight for three days of food, drinks, and ice. A 65-quart gives you room for a big bag of ice, raw meat, veggies, and drinks, plus extra space for unexpected items. For car camping, rotomolded coolers keep ice longer but are heavy; a standard cheap cooler works if you pre-chill everything and keep it in the shade.