My dad always said that it should fit an 4x8 sheet of plywood laying down. Also that if you don't need to haul 4x8 sheets of plywood on the regular then you probably don't need a truck.

I've never owned a truck, and there honestly hasn't been mant times when my Mazda 3 wont do. . Six drawer dresser - check. Queen size mattress-check. Hockey bags and kids- check. Visitors from the airport and luggage- check. Ikea furniture-sometimes sticks out the back.