Knee-high to a toad, recorded in 1814, was the original of this Americanism, and knee-high to a mosquito as well as knee-high to a frog appeared before it came on the scene some thirty-seven years later. But knee-high to a grasshopper has outlasted all the others, including the later knee-high to a duck. It is generally used in comparisons, emphasizing youth, smallness, or remoteness in time.
There are some 120,000 varieties of grasshopper, and to be literally knee-high or tibia-high to one a person would have to range from one millimeter tall to a little more than an inch tall.
The Utes were called the Grasshopper Indians because they used grasshoppers for food in dishes like grasshopper pie.