The size of the portable refrigerator you need depends on the length of your trips and the number of people you need to cater for. For solo campers or weekend getaways, a 20-30 liter fridge is usually sufficient. For longer trips or larger groups, consider a 40-50 liter fridge. If you need to store food for extended periods or have multiple people, a fridge with a capacity of 60 liters or more might be necessary. Always consider the space available in your vehicle and the power requirements of the fridge.