https://bugs.winehq.org/show_bug.cgi?id=56285
--- Comment #3 from Stavros Kokkineas skokkineas@protonmail.com --- (In reply to alexandros.frantzis from comment #2)
If the screen is scaled then, at the moment, you will need to manually apply the same scale factor in Wine for fullscreen to work properly (winecfg -> Graphics -> Screen resolution). 1x is 96dpi so 2x => 192dpi, 1.5x => 144dpi etc.
Let me know if this fixes things for you.
Yes, setting it to 144 DPI on my 2560x1600 screen fixed it. Thanx! :)