1. go to game's properties in your library
2. in the launch options, type in the box: "resize width height" where the width and height are what you want, as shown in the image
3. close and launch the game

hint: for the values, put your game in the size you want, take a screenshot and open it in paint where you can then drag select the client and see the dimensions, i'm sure there might be an easier way but that was most convenient for me

edit: make sure game is closed when you add the launch option