You don't get small during the main quest.
There are three places where your character will say "I'd have to be tiny to get through there." These are three locations used in the Bonus Quest for paid monthly members only. When your character is turned into a small blue bat, you can enter the three locations:
-- high on the Library wall between the middle chandeliers
-- the small hole atop the second cemetery crypt (next to the two vampire bats)
-- the boarded doorway at the rope bridge.
curse
poptropica
Vampire's curse is the most recent island.
You must have a membership.
kill the satan
It's the bonus quest that makes you smaller.
236
03/22/12
go to poptropicasecrets.com
the 4th easiest island is vampire curse
On the right side of the room.
On the floor by the left book case in the Library