Generally, longer risers will give you less shock and a shorter working limb, which increases stability reduces the mass of the limbs. That being said, 16" seems to be a good length, as many amazing bows are made with it. I personally wouldn't go shorter than that, as there is really no benefit (that I can see anyways). I use 22" in my design (though my bows are 68", on the long side).
Have only completed 2 bows, so someone with more experience may have a better answer/correct a misconception of mine.