Game native resolution is 500x500 pixels (I think), and it should scale up in those increments.
So as long as you window is bigger than 1000x1000 or 2000x2000 It should scale up.
Although im not sure if it works properly on bigger monitors, what's your screen resolution?
The standard 1920x1080, but since the windows taskbar and title bar is much longer than 80 pixels, it's not easy to scale it as longer than 1000 pixels manually. 1x 2x 4x buttons for 500 1000 2000 resolution might work best in this case. Or a full screen option would work quite well. Otherwise, game is quite good (except the difficulty of the puzzles) keep up the good work!