Why does the Part's size not change?

function MakeSpark(pos)
    local effect = game.ServerStorage.Part:Clone()
    effect.Name = "Effect"
    effect.Anchored = true
    effect.CanCollide = false
    effect.Material = Enum.Material.SmoothPlastic
    effect.Locked = false
    effect.TopSurface = 0
    effect.BottomSurface = 0
    effect.BrickColor = Colors[1]
    effect.Size = Vector3.new(8, 8, 8) -- This line, doesn't work
    effect.CFrame = CFrame.new(pos)
    effect.Parent = game.Workspace
end

function HitEffect(pos)
        local thread = coroutine.create(MakeSpark)
        coroutine.resume(thread, pos)

end

Should make a 8x8x8 part appear.