The skelani_length array contains the lengths of each body part (there's also skelani_shoulderwide and skelani_hipwide that adjusts how far away from the spine the arms/legs are anchored).
For just a one-off character you could make a custom NPC Skelani Setup script that changes these values, if you want to change a lot of characters you might want to duplicate skelani_init_limb_lengths_humanoid and use that when setting up the smaller characters (or, edit it directly if you want every human to be smaller)