The pie charts below show the ‘ideal’ DS keyboard size that adult pianists would most likely choose if they were widely available and cost was not an issue. They assume the pianist wants access to practically any repertoire, or in other words, not be constrained by hand size.
Today’s conventional keyboard has an octave width of 6.5 inches (DS6.5). The two smaller sizes, DS6.0 and DS5.5 have octave widths of 6.0 and 5.5 inches respectively.
Children are not included here!