How do you make your avatar skinny on Roblox?
To customize your character size, click “Avatar” in the Roblox website's sidebar to go to the Avatar Customizer. You have to be R15 to change your scale, so make sure that your Avatar Type is set to R15. Below the Avatar Type is a section titled “Scaling” with a sliding scale for Height and Width.
Character Scaling - Roblox Blog
blog.roblox.com › customize-your-avatar-with-r15-character-scaling