You get the shrink potion from the black widows dream.
There is no code that is part of the quest on Super Villain island.
It says what the new super powers are for each villain, and possible weaknesses.
You will find the shrinking potion on one of the three paintings that you have to clean in the Black Widow's dream. It is at the upper left on the first painting (Picasso's Three Musicians).
A super hero
a super villain should live in the deasert so no body can see or hear about him
super hero
joker
No
There are 4, one from each super villain's dream. (see related question)
The next villain is Speeding Spike in the subway tunnel. (see related question)
to shrink at will and have super strenght
* There are 6 super villains to be defeated on Super Power Island. (2008) (see related question) * Four "super villains" (not actually super) appear on Super Villain Island. (2012) (see related question)