To do this you need to get the code for the beakers in the lab by riding the little green car on the lake floor around to the window outside the lab and noting the proper proportions of each color liquid.
The code for getting big in the green lab is located at the top of the stairs by the window in the lab.
Once you are big look at the statue on the right side of the room near the second pillar. The statue is the one with the h looking thing on his head. Press the button in the middle of the h.
A ladder should come down from the ceiling.