How to change the size of only 1 surface of a part ?

I've added in my game a script. When we press the correct key it is supposed to create a beam in front of you that deals damage but there is a problem : I dont know how to change the size of only 1 surface of a part , without using Resize() caus it doesnt work with .Touched event. I tried using Vector3 but im not totally at ease with it. I found how to increase the size properly but the size does not increase in front. For example, if i chose to increase the Front size with Vector3, it also increase Back size. I though about modifying the position but i dont know how to do it.

So, i know that wasnt very simple to understand so, this is what is would like : When it grow (by the loop in the code down below) i would like the beam to always be at the correct position (the beginning just at my wand's position and the end at where i aimed the mouse)

Here is the code :

game:GetService("ReplicatedStorage").Tool.OnServerEvent:connect(function(player, position)
    -- the position variable is the position where the player clicked with the mouse
    local tool = game.Workspace[player.Name].WandTool

    local beam = script.Parent
    beam.CFrame = CFrame.new(tool.Emitter.CFrame.p, position)

    for i = 1,20 do
    beam.Size = beam.Size + Vector3.new(0,0,1) --This increase the size of both side (Front and Back, Top and Bottom, etc.... it depends of what side i chose to modify)
    beam.Position = beam.Position + Vector3.new(0,0,1) -- It doesnt do the right thing
    wait(0.1)
    end
end)