Editing Double Awning Window

[Sydney23]

I am putting in 'Double Awning' windows, but by default the two panes are each half the height of the window. I need the top (opening, hinged at top) pane to be a lot smaller height-wise, maybe 25% of the whole. I've searched the manual and this forum, have tried the Windows dialogue and handles in 3D Vector view, but no joy.

Think I must be missing something very basic here?

Andy