Five Cubic Feet should be sufficient for a family of six.
60 feet
3 square feet.
"square foot" is singular, "square feet" is plural. In other words, if you are referring to one 12" by 12" area, (144 inches or less), square foot is appropriate. However any dimension greater than 12" by 12", (that is, 144 total inches once the multiplication is complete), square feetis the nomenclature which is germane.
2 feet because 2 feet equals 24 inches and 24 is bigger than 23 so your answer is 2 feet.