I was going to ask the same thing. I have a 1080 desktop resolution and it only fills half the screen in a window. Anyone with a higher res than that and it would be barely playable.
And before anyone suggests lowering desktop res, not only does that sometimes make the image blurry, it can mess up positioning of desktop icons, as it would in my case.

Based on the information from Dad’s Gaming Addiction video, http://gtm.steamproxy.vip/app/552030#scrollTop=2666 – DGA Plays: Artillerists (Ep.2 – Gameplay/ Let’s Play), pressing ‘Enter’, ‘F11’, or ‘Esc’ will switch between Fullscreen and Window Mode.