Just start the game with the following command-line arguments:

With X and Y being the horizontal and vertical window size respectively.

Based on my testing, the game supports 16:9, 16:10, and 4:3 resolutions. The most common resolutions using these aspect ratios are:
16:9 - 1280x720, 1366x768, 1600x900, 1920x1080, 2560x1440.
16:10 - 1280x800, 1440x900, 1680x1050, 1920x1200.
4:3 - 800x600, 1024x768, 1280x1024, 1600x1200.

So e.g. your command-line argument will look something like this:
-window-mode -screen-width 1600 -screen-height 900

You can leave out the '-window-mode' part and switch between windowed and full-screen modes by pressing Alt+Enter.


Right-click on Saboteur! in Steam > Properties > Set Launch Options.