Imho, it's subjective...many factors affect the length of the 'perfect' keris blade. To me, it differs for each individual, a personal feel...
In the olden days, the form of the blade, the martial arts practiced and the physical size of the practitioner does influence what is deemed to be a perfect length. For the empu, the date/time of birth, occupation...etc which does influenced the end result.
Besides this, regional differences also affects the length of the blade. For example, in general, the sumatran blades are shorter than the javanese blades (of course there are exceptions and not considering the keris panjang). Therefore, this question would not get a single definitive length answer.
However, my personal preference based on silat form practised would be 12" - 13.5", Malay/Bugis blade form preferred.
