White Rabbit

This Fabric/Quilt mod adds a food and a drink that change the size of the player that consumes it.
 
Drinking Pishsalver will make you smaller (half your size each time you drink it by default) and eating Upelkuchen will make you larger (double your size each time you eat it by default).
 
The items can also be put in a Dispenser which will resize any entity in front of it when activated.
 
Recipes can be unlocked in the vanilla recipe book once you pick up Sweet Berries.
 
Several settings are configurable, including the duration of the resizing effect, the resizing multiplier for each item, the minimum and maximum scale, and the time it takes for your size to change.