Reaching as high as the knees. Frequently in jocular phrase (originally U.S.) knee-high to a grasshopper (and variants), i.e. very short.

(a) adv. = shoulder-height adv.; (b) adj. that is up to one's shoulder in height.

(a) as high as a horse; (b) too high for a horse to jump over; also figurative.

placed at such a height as to be reached by a stride.

attributive passing into adj. Of or pertaining to treetops; in the tops of trees. Also figurative.