Has anybody mentioned yet that the piano has an almost unique attribute in that the musician does not play on his own instrument when he performs? If someone regularly plays on his own custom-sized or smaller sized keyboard, and then tries to perform on a piano that has standard width keys, I cannot imagine that he'll instantly adjust. Everything will feel different. I once traded violin and viola with a family member who needed to borrow my violin for a week (long story). Adjusting back to different distances was not instantaneous. That is my main frame of reference.
As a solution to this, it is my understanding (correct me if I'm wrong) that the action (keys + hammers) can be removed as a single block. See the video for an example:
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Why can't a music hall simply have different actions for the same piano, each action with different sized keys, and use the action with the preferred key size for a given performer? Since the piano gets tuned before a performance anyways, it should be a relatively small matter for the piano technician to swap out the action at the beginning of the process.
So basically, rather than making a new piano, instead the same piano case and string assembly would be reused, but different-sized keys could be made (with larger/smaller wood blocks on the sides so that it would still fit the same piano case). An issue may be that the hammers will be somewhat diagonal (i.e. not in a straight line with the keys, but the hammer arms in the lower register would be diagonal toward the left, the ones in the upper register would be diagonal toward the right, to line up with the strings), but I don't think that's insurmountable. Then it would be a matter of the performer's choice.
I personally can only reach a 9th by collapsing the bridge of my knuckles, so while it's something I *could* do, it's not something I can do *comfortably*, and I hit the notes in between if it's on the white keys. Thus, my functional reach is more or less limited to an octave. If a piano with a smaller keyboard were available, I wouldn't mind trying it out. It would make some pieces substantially easier in terms of reaching for different notes.
People should keep in mind that the keys are only the interface, and some of their design parameters (such as size) are not intrinsic to the piano itself. It's like a computer keyboard or a car's steering wheel (whose height can be adjusted to suit different drivers). It's almost universally acknowledged that everyone's hands are different, with different finger lengths, finger widths, palm sizes, etc. Why should there be a single size for a keyboard that everybody should adapt to? It's as if everyone had to wear the same size pants or something.
Some people have brought up famous pianists with small hands, but that's somewhat of a red herring. There are shorter people playing basketball too, but being taller is definitely an advantage; according to wikipedia, the average professional basketball player's height is about 6 ft 7 in, which is far outside the norm of the general population:
https://en.wikipedia.org/wiki/Height_in_sports#BasketballJust because there was a 5 ft 3 in Muggsy Bogues or a 5 ft 5 in Earl Boykins playing professional basketball doesn't mean that the typical person of those heights would be a competitive player.
I don't think it's unreasonable to surmise that if the piano keyboard were scaled to better suit people with different sized hands, then playing piano would become more attractive to a larger segment of the population.
Topic: Thoughts on new narrower ergonomic piano keyboards to improve playing.