A 5x10 storage unit is the size of a small walk-in closet.
It fits furnishings of a mid-sized bedroom, including a queen-sized bed, a dresser, a TV and several small to medium-sized boxes.

A 10x20 storage unit is the size of a standard one-car garage.
It fits the contents of a multi-bedroom house, including several oversized items like a couch, a refrigerator, a washer/dryer and a dining room set or several large boxes.