Here is my understanding from what I have read
The top of the window is already in the correct place as it matches the rear windows
you need to lower the bottom part to match the rear windows ( I already removed the steel part that is in the way) so all you would need to do is cut the aluminium skin lower.
My hardtop sides are apart right now so I can measure the opening if you would like.
Just a reminder you will want to weld the lower corners as they have hairline cracks (every liftgate I have had so far needed this done)
going out to the garage now to measure the hole for you
I have 2 liftgate glasses, and a bunch of other glass (alpine, tinted fixed side, tinted liftgate, tinted rear small windows) let me know what you want.
Just checked my hardtop and the opening for the little rear side windows is 15.375
I think it would look best if you make the opening in the liftgat the same size.
hope that helps