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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:

vextor -show-screen-selector

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