Hello
I think I managed to resolve the Issue with the Screen being Resized to the Top Left Quarter of the Screen by following these Steps:
1 Open Registry Editor (as Admin) to the following path:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\GraphicsDrivers\Configuration\
2 Delete all entries in this Path.
3 Restart PC.
4 After Restart, re-open Registry Editor to:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\GraphicsDrivers\Configuration\
There should be one Registry entry.
Click on Tree Arrow, then Click on Second Arrow to View the Values.
By clicking on each 00 the values will be displayed on the right side of Registry Editor.
Note that there are some Value Pairs that match the Resolution of your Display.
5 Switch Off AVR and then Switch it On again.
6 Open Registry Editor to:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\GraphicsDrivers\Configuration\
There will now be a Second Entry.
This is a Dummy Monitor Entry created by Windows for when a Monitor is not present, i.e. when I Switch Off the AVR.
It has the same Value Pairs as before, but with incorrect Resolutions for the Display.
In my case the Values were for 1920 x 1080, hence why PlexHTPC was shrunk to a Quarter Screen as my Display is 3840 x 2160.
7 Change the values in the Dummy Monitor Registry Key to match the Real Monitor Resolution, i.e. 3840 x 2160 rather than 1920 x 1080..
8 Restart
I have not had the Issue with PlexHTPC being Shrunk to Quarter Screen Size since I made the above changes.
Thanks
Kevin