Size matters!

[David]

Just one example Jussi. Test by means of thumbs from A.H. Hill's chapter in Keris and Other Malay Weapons:

"Hold the keris towards you. Grasp the blade with the right thumb and forefinger just above the ganja. Place the left thumb alongside it, then the right and so work up the blade until the tip is reached. It is lucky if the last thumb width is the measure needed to reach exactly the extreme point. Several sequences of special words have been recorded, the omens then depending on the number of thumb widths needed to reach the end. Less than a thumb width counts as one full width, and as before the omen is taken from the last word uttered."

Of course this method does not determine whether your keris should be large or small necessarily as a good omen word can fall on various lengths. It also works deferently with different people since everyones thumb widths will be slightly different so it is meant as a way to determine whether a particular keris is right for a particular person.
Size of keris is also goinfg to vary dependent upon the exact culture it comes from. In Bali keris tended towards a longer and more robust form.
There are just too many variables to make any definitive statement on what is the optimal size for a keris.