when a person stands next to the barre, it should be about elbow high for that person.
Hope this helps.
Most ballet barres are fixed at 32 to 46 inches from the floor.. Many ballet schools will set up a double barre, one at 32 inches and one at 46 inches. I helped set one up years ago for a dance school. I seem to remember the instructor telling the students to choose the barre closest to the height of the waist. But that was ballet, not the barre exercise program.