In the past I have tried older games in PlayOnLinux that do not support widescreen resolutions. But the virtual desktop will resize to resolutions like 1024x768 or 1280x1024 and appear windowed. The only way to make the game appear fullscreen is to set the Linux Desktop resolution to match the game.
The easiest way to do this is by adding:
xrandr -s 1280x1024
in the PlayOnLinux Arguments for that game. It was working perfectly, but lately I've noticed that the command doesn't work and the resolution does not automatically change.
Mint 17.3 64-bit
GeForce GTX 950
Wine 1.8 (depending on game)