Occasionally, players on Steam encounter an issue where their game is opening in an unusably small window that can't be resized. This issue can be easily fixed by editing Steam's launch options for Bit Heroes Quest.
To access launch options, right-click on Bit Heroes in your Steam library and navigate to the Properties menu. Then, enter the following text in the launch options field of the General sub-menu.
-screen-width 1920 -screen-height 1080You will be able to adjust the window to your preferred size once you have launched the game. Alternatively, you change the values here to match the resolution of your own screen.