The game has a resolution of 640x360, but can be up-scaled to fit to your monitor and take up the entire screen.

Navigate to the game's install directory and find the file called "Game.ini" and then open it with notepad or a similar program.

Find the section with "[Fullscreen++]"

You can now replace that entire section with any of these to suit your desired style:

Windowed:

[Fullscreen++]
Fullscreen=0
FullscreenRatio=0
WindowedRatio=1

That will set the game to be a small window.

[Fullscreen++]
Fullscreen=0
FullscreenRatio=0
WindowedRatio=2

This will set the game to be a mid sized window.

[Fullscreen++]
Fullscreen=0
FullscreenRatio=0
WindowedRatio=0

This will set the game to be a very large window - taking up most of the screen.

Fullscreen:

[Fullscreen++]
Fullscreen=1
FullscreenRatio=3
WindowedRatio=0

This will set the game to be exclusive fullscreen, but the game graphics will be very small on screen, surrounded by black borders - preference only.

[Fullscreen++]
Fullscreen=1
FullscreenRatio=2
WindowedRatio=0

This will set the game to be exclusive fullscreen, but the game graphics will be mid sized on screen, surrounded by black borders - preference only.

This will set the game to be exclusive fullscreen and take up the entire screen.

[Fullscreen++]
Fullscreen=1
FullscreenRatio=1
WindowedRatio=0

Make the changes you want and save it. That's all there is to it.

If you want to keep those changes permanently, simply set the file to be "Read Only" and you're all good.