Post office boxes come in a variety of sizes. A size 1, or extra-small box, is designed to hold 10 to 15 regular-size letters. On the other end of the spectrum is the Size 5, or extra-large box. It will hold flat-rate boxes and other parcels. You can find sizes in between these two as well that will hold large envelopes or even shoe boxes. You should select the box size based on the types of items you expect to receive.