How do I measure the length of the side of a cylinder?

You just need to find the circumference of the circle.
C=2πr or C=πd

The term you’re looking for is circumference. Given that you know the diameter working out the circumference is trivial

@bobbinhood & @Lightlyseared Circumference! That’s the word I was looking for! Thanks guys! And thanks for the formula ;)

Couldn’t the cricumference be different from the length from top to bottom?

@janbb Absolutely. But the details gave me the impression that the OP was looking for the circumference, not the height.

If the diameter is 6,8 cm, then the paper has to be at least 21,362831 cm long to wrap all the way around the bottle.

@bobbinhood Yes, the question and the details don’t really match. You should need to know both actually to cut the paper.

Don’t forget to add the overlap where the ends of the paper join using tape, glue, etc.

This is a piece of pi. Are we doing your homework?

@cazzie No, this is not my homework. Just a normal question I had. But I got it fixed now. Thanks everyone