You can change the resolution manually in user settings file, it should be located in: C:\Users\<your_user>\AppData\Roaming\WizardsOfTheCode\Soulash\data\user_settings.json
You can change the "viewport.resolution.w" to 1600 and "viewport.resolution.h" to 900 there. Remember to change the file while the game is not running, because it will overwrite the file when you exit.
Let me know if you need any more help.