Is the resolution different in game since u upgraded?
Thinking try and change the resolution with your keyboard if you can and see if that fixes it. tab moves thru options. try UI resolution and game.
Have you installed any other games?
framework or direct x maybe missing something.
Win 10 isn't really updated properly yet so its like as if you have to go and install everything again like you would fresh. I know it picks up most of the install files and what not on upgrade but I found if I didn't install it again it didn't work...properly.
Also is your build up to date as of right now? I know there was a few updates yesterday.
"The only similar setting to Windows 8.1 is Windows 10 "Change size of items", and I have tried almost all different combinations"
Not sure what you're saying here...
change icon sizes?
"I have just completed a new full install"
Of dayz or windows?
after deleting those files all you have to do is verify game cache. Not reinstall. But it's obviously nothing to do with game settings if you did reinstall.
I doubt these guys can do much for you.