You can have multiple stacks of dye to apply the resizing multiple times, so 3 stacks of Cyan Dye will make you 1/8 size, and 3 stacks of Magenta Dye will make you 8 size. Having a stack of Cyan Dye in your inventory will make you half your size, and a stack of Magenta Dye will make you twice your size. (You can drink Milk or the Potion with the opposite resizing effect to change your size back.) Potion of Embiggening makes you 4 times normal height, Ensmallening makes you 1/4 normal height.
Riding an Iron Golem with String! How to Change Sizeīrewing recipe (Gulliver v0.10 for Minecraft 1.4.5 & later):
There are no new blocks or items, just two Potions.