Based on the evidence to date, it appears that we need at least three keyboard sizes. These are today’s conventional ‘large’ size (6.5 inch octave), plus the DS6.0® (6.0 inch octave, approx 15/16) and DS5.5® (5.5 inch octave, approx 7/8) of the current width. David Steinbuhler has named these sizes as DS6.5 , DS6.0® and DS5.5®. (DS stands for Donison-Steinbuhler.)
The DS5.5® is particularly important in that it removes the advantage that men have over women due to average hand span differences. It is also suitable for children. (See: Pianist’ hand spans vary greatly!)
The DS6.0® is a potential ‘universal’ size that would suit many private homes and public venues very well. A pianist can adapt to it almost immediately. This size is immediately popular with many who try it even for a short time.
There may also be a market for an even smaller sized keyboard designed mainly for children, with a 5.1 inch octave (DS5.1®). Already, some adults with very small hands have chosen a 5.1 inch octave keyboard for themselves from the Steinbuhler company.
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.
Children are not included here!