"Must" is not a measurement term. It's likely you meant to ask:

How *long* is 5 cm?

5 cm is a very short length. Here are some comparisons:

* About the width of a standard paperclip.

* About the width of your pinky finger.

* About one-fifth the length of a standard ruler.

Let me know if you'd like to know how 5 cm compares to something else!