You should get a letter from the caretaker that says "come meet meet me" where she tells you the recipe . Then just walk to the caudron and the icon for brewing the shrinking potion should appear just like the other ones.

Hi there When you enter

In the cave , look up on the ceiling. You should see an ice patch , hit it with fire spell and a key will fall down

Mix blue and orange potion acording to the recipe you get when you get this objective. Just walk to the cauldron and you should see "mix shrinking potion " the hit "brew". Hope it helped.

You don't. Transform into a cat and walk up the wooden planks up until you reach a tunnel, then walk down on other planks onto the other side . ps: you must do that when you return aswell.