The variation in hand size between men and women is significant, and it’s even greater when children are included.
The available data indicate that, on average, adult male hand spans (thumb to fifth finger) are about one inch (or 2.5 centimetres) greater than adult females. This means men can reach, on average, more than one extra white key on the current keyboard. See: http://smallpianokeyboards.org/hand-span-data-recent-australian-study/
An important benchmark separating ‘small’ from ‘large’ hands is a span of 8.5 inches. Up to this point, the pianist struggles to play a tenth, and more importantly, fast passages of octaves and large chords can be uncomfortable and involve pain or tension. From the available data, it is estimated that about 76% of adult men have hand spans that can reach 8.5 inches or more – this leaves about 24% of men (a not insignificant proportion) who cannot play a tenth. For women, it is the reverse, as it is estimated that only 13% of adult females have hands large enough to play a tenth with 87% unable to!
Some races with generally smaller hands are further disadvantaged. Based on a recent Australian study (presented to, and published by the Australasian Piano Pedagogy Conference 2015), it appears that over 90% of female Asian pianists have spans below 8.5 inches. Children do not have the advantage of learning on a keyboard that suits their hands. Older people may find that their span is reduced with age.
A smaller study of the hand spans of children indicates that there is a significant overlap between the spans of adult women and those of young children (under 12 years). In other words, a significant proportion of women (almost one third) have child-sized hands.
The photos below compare ‘average’ female and male hands:- the male hand is very relaxed playing octaves and can accomplish legato ocatves. The male 9th is equivalent to the female octave, with thumb near the front of the keys, reduced power due to the flat hand and fingers outstretched, also producing tension in the hand during extended passages. The female 9th is equivalent to the male 10th – just playable on the front edge.
For further information, see: http://smallpianokeyboards.org/how-many-adults-have-small-hands/