You just have to select configure in the main
playonlinux gui, then select the wineprefix with the game in it when the configuration window loads. Then in the configuration window choose the wine tab and select 'configure wine', which will launch the familiar
winecfg. In that
wine configuration menu, go to the graphics tab and choose 'emulate a virtual desktop' and select the resolution you require and apply the settings.
Very often you can choose a windowed option in the game itself rather than using the
wine virtual desktop.
All the menus and windows I refer to in the first paragraph above are shown in the screenshot below, where I show how to configure a virtual desktop for a
For more information, please refer to the documentation at the official site, and if you wish to backup your game and its settings in the
playonlinux wineprefix, see my answer here.