Get the absolute height of the user's screen
Multiply it by a decimal between 0 and 1 (0 = 0% of the screen, 1 = 100% of the screen) depending on the scale you want it at.
Set the offset size to that number
And for the position, you want that number/-2

Position = 0.5,number/-2,0.5,number/-2
That is, if you want the screen centered. You can change the 0.5 coordinates to shift its offset.

You have to update this every time the absolute size of the GUI changes.