The most recent leak with information regarding the screen size of the upcoming iPad Mini 6 mentions an 8.38-inch display. This dimension was spotted in an Amazon Japan listing for an iPad Mini 6 screen protector. Sadly, the page has been removed from the website and is no longer available.
https://www.notebookcheck.net/New-iPad-Mini-6-rumor-October-launch-8-38-inch-display.561363.0.html