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?