Based on the evidence to date, it appears that at least three sizes are needed: specifically, the current ‘large’ size (6.5 inch octave), plus the DS6.0® (approx 15/16) and DS5.5® (approx 7/8) of the current width. These sizes have been named by Steinbuhler as DS6.5 , DS6.0® and DS5.5®. (DS stands for Donison-Steinbuhler.)
The DS5.5® is particularly important in that it largely removes the advantage that men have over women due to hand span differences, and is also suitable for children. (See: Hand span data)
The DS6.0® is a potential ‘universal’ size that would suit many private homes better than the current ‘large’ size. It also requires virtually no adaptation time from the current size, so is immediately popular with many who try it even for a short time.
There may also be a market for even smaller sized keyboards mainly for children, for example, with a 5.1 inch octave (DS5.1®). These could be provided in the form of digital keyboards or as low cost upright pianos. Already, a small number of adults with very small hands have chosen a 5.1 inch octave keyboard for themselves from the Steinbuhler company.
Manufacturers of acoustic and digital pianos producing products of different sizes will benefit from increased demand. Clearly, the number of keyboard sizes available in mass produced pianos and keyboards will need to be limited. (However, the option of custom-made ‘other’ sizes can also be available, as it is now to a limited extent.)
It will be important for manufacturers to adopt standardized key widths for keyboards of different sizes and label them in a consistent way for the benefit of those purchasing or playing and to reduce confusion in the market place. Queries about use of the DS labels should be directed to the DS Standard Foundation Inc. : http://dsstandardfoundation.org
For grand pianos, manufacturers should provide consumers with the option of purchasing more than one interchangeable action/keyboard to provide maximum flexibility for schools and conservatoriums, piano competition and concert hall use. But consumers should of course have the option of choosing just one keyboard for a grand piano. (Currently, a pianist wanting a grand piano with DS5.5® keyboard will first have to acquire a grand piano with conventional keyboard, then purchase a second retrofitted action/keyboard from the Steinbuhler company.)