Right click the game --> Properties --> Set Launch Options:


You can choose your own resolution (this is a Unity launch parameter, if you want to know), or just use the resolutions provided in-game.The launch option will always execute at launch, thus, the game will always start with that resolution (Personally I prefer the smaller size).

Alternatively, these are some common monitor resolutions you can paste if you want a window of this size, just make sure it's 16/9

Remember, it will always start up with that resolution if you keep the launch option, even after you change it in-game.