For top-freezer-type refrigerators, whose capacity may range anywhere from 10 to 25 cubic feet, 18 cubic feet would be about average and is a typical size. However, a refrigerator of this size would be considered on the small side for a bottom-freezer-type refrigerator.
A refrigerator with a capacity of 18 cubic feet would likely provide adequate food storage for a family of three or four at most. Keep in mind that for a refrigerator of this size, the freezer takes up about 5 cubic feet, of which an icemaker takes up 1 cubic foot. The remaining space in the main compartment includes bins, shelves and drawers.