Expanding the part bigger just on a side but it expands on both sides?

Hi, so i would like to know how to make a part bigger, BUT on only a side because whene i try expanding the size it expands on the 2 sides

any help? btw heres the code

    game.ReplicatedStorage.tornadoCloud.Parent = workspace

    game.ReplicatedStorage.Sky.Parent = game.Lighting
    for i = 201, 989.5, .05 do
        workspace.tornadoCloud.Size = workspace.tornadoCloud.Size + Vector3.new(0, 0, i)
        wait(.5)
    end

any ideas?