Answer:
Centimeters or milimeters.
Explanation:
The answer depends on the size of the box.
The metric system base unit for length is the meter. Centimeters and milimeters are thus also part of the metric system.
If the box is a "regular-sized" box that we use to put stuff in (books, presents, etc), then centimeters would be the most appropiate.
If we're talking about those tiny boxes that are printed on paper (for example in some notebooks) then milimeters would be the most appropiate.
