So this is a mod request then? I don't think this is possible in current version, might be mistaken. What do you want the shrinking to do? Just make you smaller so you fit in a 2x2 block area or something? Use morphball tech.
There are people who make a tech with lua coding that allows them to shrink to half of there size and smaller so they fit into 1x1 blocks and then they can gradually make themselves bigger, shrinking is what I request most.
