Roughly the size of a small closet. Ideal for storing seasonal items, boxes, small furniture (like a chest of drawers or small desk), and sports equipment. Think of it as holding the contents of a walk-in closet.
Comparable to a large walk-in closet or a small garden shed. Can typically hold the contents of a studio apartment or a dorm room. Suitable for a mattress set, a few pieces of furniture, and several boxes.
Similar in size to a large bedroom. It can usually hold the furniture from a one-bedroom apartment, including a queen-sized bed, dressers, a sofa, and many boxes.
Roughly the size of a large bedroom or a small one-car garage. It's suitable for the furnishings of a two- to three-bedroom house, including appliances, full furniture sets, patio furniture, and numerous boxes.