Taking for granted that a normal keyboard is perpendicular to the floor,
or even parallel
-- but can we really take that for granted?
On my piano the keys are pretty well horizontal when pressed
, but of course because they are pivoted, then once released
they rise more at the front than at the back, giving a slope which I estimate to be 2 degrees. When I sit at the piano, my line of sight to the middle of the keys straight in front of me slopes down at about 45 degrees to the horizontal. This means that the keys, by virtue of my viewing angle, are already shortened to 70.7% (cos(45 degrees)). By sloping 2 degrees, the shortening is instead to cos(47 degrees) which is 68.2%. I don't think the difference is enough for a significant psychological effect. Your brain knows how long the keys are, and isn't fooled by the viewing angles.