Typically the suggested size for speaker height (for fronts) is to have the tweeter at about the same height as the ear position. You could assume that when a person is sitting in a sofa/chair that ear height would fall between 2.5-4' level and hence, the rather consistent heights of most speakers in the 30some to 40some inch range.
Consider a speaker (3 way) that is 7 feet tall with the tweeter at the top....

"Those who preach the myths of audio are ignorant of truth."