1. There is a job you can select called "Restaurant Owner"
This job allows you to use ingredients to create food items.
Food items all have different effects when created and the effects are randomized with what you make. There are ones that make you bigger or smaller and if you interact with them it will change your size.
2. Potions can also make you smaller or larger and can be acquired from Quests, Lightening Quests, or can be purchased with Quest Tokens
3. If you are in a gang and have a gang perk called morpher, you can also change your size using /size from .5 to 1.5 depending on if you are on a job that allows it
.png.dd2b432ae1812ea1fd2f38a267de1517.png){kind=logo}
Spektor's post in how to change size? was marked as the answer