Hello! I’m not really experienced with changing humanoid properties, but I’m pretty sure you can use HumanoidDescription and change the scale settings!
As @wc3u said Scripts inside StarterCharacterScripts are placed inside each player’s character, so you can simply get the character with script.Parent. You can use ApplyDescription to apply all the changes to the body.
Place a ServerScript inside StarterCharacterScripts and place this script: (Make sure to change the numbers to whatever you want them to be!)
local Character = script.Parent
local Humanoid = Character.Humanoid
--Set the humanoid properties:
local humanoidDescription = Instance.new("HumanoidDescription")
humanoidDescription.DepthScale = 0.1
humanoidDescription.HeadScale = 0.1
humanoidDescription.HeightScale = 0.1
humanoidDescription.ProportionScale = 0.1
humanoidDescription.WidthScale = 0.1
-- Apply them to the humanoid:
Humanoid:ApplyDescription(humanoidDescription)