Resize Increment in parts?

I'm trying to script parts to EXACT sizes and I'm having issues with ResizeIncrement in Part Its automatically set to 1 and I can't figure out how to change it cause everytime I try it errors and says "can't set value"

Trying to get a new right arm and the size is automatically changed to Vector3.new(1,2.4,1)

Char=game.Workspace.blowup999
function weld(a, b)
    local weld = Instance.new("ManualWeld")
    weld.Part0 = a
    weld.Part1 = b
    weld.C0 = CFrame.new()
    weld.C1 = b.CFrame:inverse() * a.CFrame
    weld.Parent = a
    return weld;
end
c=Char["Right Arm"].CFrame
Char["Right Arm"]:Destroy()
right=Instance.new("Part",Char)
right.Size=Vector3.new(1,2,1)
right.CFrame=c
weld(right,Char.Torso)