Well it depends! some are really small, like the ones that go around your wrist. Others are longer, for example putting a string up from one place to another to create a clothes line. Normally a piece of string is as long as you measured it in the first place, so if you wanted to cut a 30cm piece you would measure it... so the string would be 30 cm.
Another example would be if you needed a 80 cm piece of string.... you would measure the string piece you were cutting and then you would cut it at exactaly 80cm...so the string would be 80cm.