I installed windows beta (x64) 18.104.22.168 on my Surface Pro and the windows were enormous.
I tried re-installing a couple of times but the windows for Scrivener were so large that navigation was futile.
I un-installed the x64 Beta and installed the x86 version and I was good to go.
The application runs fine and the window size is what I expect.
Not sure if this has already been reported. Maybe it is a quirk of the Surface Pro?