1. I was just knee-high to a grasshopper when I first went fishing with my father. I couldn’t have been more than five years old.
2. Look how small these pants are! I must have been knee-high to a grasshopper the last time I wore them.

The expression suggests that the person is only as tall as a grasshopper’s knee and is therefore very young. It is often used with a facedown, open hand to indicate the young person’s height at the time.