Sadly, there's no option in the game to change resolution or to force windowed mode from the start. However, there's a simple way to play this game in windowed mode at a custom resolution. Here's what to do:

• Right click the game title from your steam library, and select "Properties..."
  
• Under "General", click "Set Launch Options..."
  
• Now type the following in the box:
  -screen-fullscreen 0 -screen-width 2560 -screen-height 1440

This would start the game in windowed 2560x1440 resolution. For any other resolution, just pick the values you want, but note that not all resolutions may be supported. To start the game in a custom resolution in full screen, just change the 0 into a 1.

Enjoy!