hmmm well the game definitely doesn't actually have Steam Deck support right now.
I wonder why the resolution is square like that though, I thought Steam Deck was more along the lines of a 16:10 aspect ratio.
Unfortunately resolution options haven't been implemented yet, so there's nothing natively in the game that could help you.
If there is a way for you to toy with resolutions (or even windowed/not windowed) in Steam Deck, that's all I can think of.