How do you make a script where when you spawn, you spawn sized 0.5?

The reason why your current script doesn't work is because it doesn't recognize the number values. The size variable for the character parts are defined as a Vector3, so you have to create a new vector for each part.

function onPlayerEntered (newPlayer) 
    wait (5) 
    newPlayer.Character.Torso.Size = Vector3.new(0.5, 1, 0.5) 
    newPlayer.Character.Head.Size = Vector3.new(0.5, 1, 0.5)  
    newPlayer.Character.leftArm.Size = Vector3.new(0.5, 1, 0.5)  
    newPlayer.Character.rightArm.Size = Vector3.new(0.5, 1, 0.5)  
    newPlayer.Character.leftLeg.Size = Vector3.new(0.5, 1, 0.5) 
end

game.Players.PlayerAdded:connect (onPlayerEntered)

This however only resizes each part to what is defined, and doesn't exactly scale your model as you think it would. To actually do that, it would require a lot more coding which is out of my scripting knowledge.

CREDITS: CloneTrooper1019

function scaleCharacter(model,scale)
    wait()
    if not model or not scale then return end
    if not _G.ScaleCons then
        _G.ScaleCons = {}
    end
    if _G.ScaleCons[model] then
        _G.ScaleCons[model]:disconnect()
    end
    local joints = {}
    local parts = {}
    local h = model:findFirstChild("Humanoid")
    if h then
        h.Parent = nil
    end
    local function handleHat(hat)
        if hat:findFirstChild("GotScaled") then return end
        Instance.new("Flag",hat).Name = "GotScaled"
        Spawn(function ()
            local h = hat:WaitForChild("Handle")
            local m = h:WaitForChild("Mesh")
            m.Scale = m.Scale * scale
        end)
        local yInc = (scale-1)*.5
        hat.AttachmentPos = (hat.AttachmentPos * scale) - (hat.AttachmentUp * Vector3.new(yInc,yInc,yInc))
    end
    for _,v in pairs(model:GetChildren()) do
        if v:IsA("BasePart") then
            table.insert(parts,v)
            v.Anchored = true;
            v.FormFactor = "Custom";
            for _,j in pairs(v:GetChildren()) do
                if j:IsA("Motor6D") then
                    local t = {
                        Name = j.Name;
                        Parent = v;
                        Part0 = j.Part0;
                        Part1 = j.Part1;
                        C0 = j.C0;
                        C1 = j.C1;
                    }
                    table.insert(joints,t)
                    j:Destroy()
                end
            end
        elseif v:IsA("Hat") then
            handleHat(v)
        end
    end
    for _,v in pairs(parts) do
        v.Size = v.Size * scale
        v.Anchored = false
    end
    for _,j in pairs(joints) do
        local c0 = {j.C0:components()}
        local c1 = {j.C1:components()}
        for i = 1,3 do
            c0[i] = c0[i] * scale
            c1[i] = c1[i] * scale
        end
        j.C0 = CFrame.new(unpack(c0))
        j.C1 = CFrame.new(unpack(c1))
        local n = Instance.new("Motor6D")
        for k,v in pairs(j) do
            n[k] = v
        end
    end
    model.ChildAdded:connect(function (c)
        if c:IsA("Hat") then
            handleHat(c)
        end
    end)
    if h then
        h.Parent = model
        h.WalkSpeed = 16 * scale
        h.MaxHealth = 100 * scale
        h.Health = (h.Health/h.MaxHealth)*(100*scale)
    end
    _G.ScaleCons[model] = con
end

game.Players.PlayerAdded:connect(function (player)
    player.CharacterAdded:connect(function (character)
        scaleCharacter(player.Character, .5)
    end)
end)