I cant figure out how to change a players size by script?

I had a similar problem. The R15 humanoid scaling didn't seem to load in. Try adding them to the humanoid directly

game:GetService("Players").PlayerAdded:Connect(function(Plr)

    Plr.CharacterAdded:Connect(function(Char)
        local Humanoid = Char:FindFirstChildOfClass("Humanoid")

        local HeadScale = Instance.new("NumberValue")
        HeadScale.Value = 1

        local BodyHeightScale = HeadScale:Clone()
        local BodyWidthScale = HeadScale:Clone()
        local BodyDepthScale = HeadScale:Clone()

        HeadScale.Name = "HeadScale"
        BodyHeightScale.Name = "BodyHeightScale"
        BodyWidthScale.Name = "BodyWidthScale"
        BodyDepthScale.Name = "BodyDepthScale"

        HeadScale.Parent = Humanoid
        BodyHeightScale.Parent = Humanoid
        BodyWidthScale.Parent = Humanoid
        BodyDepthScale.Parent = Humanoid

    end)
end)

The values must be there for the model to exist. you are simply missing the Value parameter.

game.Players.LocalPlayers.Character.Humanoid.HeadScale.Value = #