Anyone accustomed to playing modern keyboard instruments immediately notices that the keys of antique keyboards look smaller. The older keyboards are always shorter front to back than modern ones, and they are often different in width. Considering the smaller average size of players in the historical period, it is surprising how much of a range of sizes are found in historical instruments. A 1681 double manual by Vaudry has approximately a 6" octave (1/2" narrower than modern), another French double of 1667 is just a bit wider than that. On the other end, several antique Italian harpsichords, including some by the well-known maker Grimaldi, have a 6 5/8" octave, larger than modern.
Some of the reasons for key size differences are practical, some entrepreneurial, some fashionable, some mechanical, and some due to playing style.
Historical instrument builders were a practical lot, and whenever possible liked to use measurements in whole inches, or easy fractions of them. The size of the inch, though, varied from region to region, and from time to time, so this naturally resulted in the key sizes varying from region to region. In fact, these different inch sizes have recently become a useful way of helping to determine, or confirm, where various antique instruments were built.
The famous Ruckers family of harpsichord builders flourished in the sixteenth and seventeenth centuries. Their keys were the same width as modern keys, just under 6 ½ of our modern inches for each octave of keys. In the eighteenth century the native French harpsichord builders, and players, became aware of the marvelous sound these old Ruckers instruments had and wanted to adapt them to their own time. They often would make much more money selling an old Ruckers than one of their own new ones. In fact to spread the joy a little further, it was not uncommon for them to take a soundboard out of an old one and build a new case for it, and then build a new soundboard to put in the old case - giving them twice as many "antiques" to sell!
A major problem for these French antique dealers, however, was that 45 keys had been enough to play the music of Ruckers time, but not enough for the eighteenth century French music. Their initial solution for this was to squeeze the strings a little closer together and make new keyboards with keys a little narrower than the old ones. This led to the original Ruckersí octave size being reduced to 6 ¼ modern inches, often now called the "French octave" among harpsichord aficionados. Because the 20th century harpsichord revival often used these French instruments as models to build new harpsichords, it often became assumed that harpsichords typically had keys of this size. In fact, it is not really "the" harpsichord size, but "a" harpsichord size - that of many 18th century French instruments. Not necessarily applicable to German, English, Italian, Spanish, or Flemish harpsichords, or even to French harpsichords of the 17th century as noted earlier.
Considering that average hand sizes have grown (along with our bodies) in the last two hundred plus years, this French size would feel significantly smaller to our hands than it did to the original owners.
Apparently builders also responded to the needs of individual customers in building their keyboards. A 1786 harpsichord still exists by the famous builder Pascal Taskin which was made with a keyboard with an octave width of 4 ¾"! This was built for a very small woman of the aristocracy (and not for a child, as had been previously speculated, who would have outgrown it in a few years anyway) and added to her other scaled-down furniture.
Another aspect of width is the size of the spaces in between the accidentals. For the first few centuries of keyboard playing it was apparently not part of the playing technique to place fingers in between the accidentals since these spaces were often too narrow to allow it. This fact allowed the tops of the accidentals to be made wider.
This sharp size consideration ties in with another aspect of key size: the front-to-back length. These lengths are often a result of playing style and mechanical requirements of the different mechanisms of the clavichord, harpsichord and fortepiano.
Briefly, playing techniques for the earlier clavichords and harpsichords used a more arched finger position in order to keep the finger tips in essentially a straight line as if resting on the home row of keys of a computer (or typewriter) keyboard. The keyheads did not have to be very long because if they were, it would require more of a reach back to the sharps from the home position near the front of the naturals. Sharps also did not need to be very long since the further back on them that one played the more reach would be needed to play a natural key since fingers couldnít fit in between the sharps to push down the naturals. Typically by the beginning of the 18th century keyheads might have grown slightly to 1 3/8" long and the distance from the front to the back of the key to perhaps 4" to 4 1/2", on average.
As the 18th century progressed the spaces between the sharps became roomier permitting fingers to fit between, and the lengths of the keys continued to grow. This increased length may have been of some mechanical advantage for clavichords and fortepianos, but probably more likely was a result of a changing playing technique using a more open hand position.
When building keyboards in the 20th century several of these considerations are still valid: reproducing a particular instrument along with itís keyboard, to fit a particular hand size, and to permit a particular playing style. Although it must be noted that the mechanics of these instruments do respond better to different playing techniques than that which developed to suit the mechanics and music of the modern piano. Keeping key sizes and proportions within the range of historical examples is most likely to permit these instruments to be used to maximum advantage.