Set default window size for Minecraft
Solution 1:
It's easier than in the self-answer. In the launcher, you can click on "Installations", then on the profile you want (default is "Latest release") and then set a resolution in there.
Solution 2:
Well, never mind. I figured it out.
Edit .minecraft/options.txt and set the overrideWidth and overrideHeight values to your desired values. This will be the size of the game screen itself (so the actual window will be slightly larger with your OS's borders).
Make sure the game and launcher are closed while you are editing this file, otherwise it will reset them to 0.