Maybe a mod or smtn?
Various sizes, including height (I believe) can increase when aging (drinking a maturing potion). The same sizes can shrink from a youth potion.
Lots of things, including skin covering, can change from mutations.
As for changing either of these things to a specific setting of your choice, only way I know how to do that at present would be to modify the save file directly.
how do you do what?
potion: make potion, pick person you want to drink it, and go to item - use
mutations: cast mutation spell or build up the slaves toxicity and mutations will happen automatically
save file: open it with any text editor and change it however you want (make a backup of the file first in case you break it)