There's no in-game UI for it, and I honestly have no idea if unexpected aspect ratios totally break anything, but because it's built in Unity you can launch it with the standard Unity command line parameters:
lets you choose a size, windowed or not, etc. Don't try and customize controls in this startup screen though, the input system depends on certain values and all input customization should be done in-game.
You can also start with a preset window size, e.g.
vextor -screen-fullscreen 0 -screen-width 1024 -screen-height 768
See the full list, under Unity Standalone Player command line arguments