Full screened for me. However, if the game automatically upscales to close to full screen, I'll go with that. I did a game jam, where I failed to make it automatically upscale, so the big complaint was "can't read settings in tiny window, to find full screen, 1 star."
Viewing post in Do you play in Fullscreen or Windowed?
Yeah, that's the solution I'm thinking about, so far games that I had developed in the past, in fullscreen, changed the desktop resolution to match the game. But, the other way around seems to be the best solution.
However, I simply cannot allow the game to just upscale, because it may get heavier the higher your desktop resolution is, for instance, if you have a potato GPU but a 4k monitor, the game will run like crap, so the solution I have is to run the game in a smaller res and just upscale at the end after all rendering is done.
With full screen, I see a lot of Steam discussion about "game is just a black screen" because their monitor can't handle full screen. So I don't recommend starting in full screen. Not only that, but using cloud to keep a full screen setting, might not be good, because people get new computers that might not work well with full screen.
If you do upscale, I highly recommend increments of 50%. Keeps it from looking awful, or stretched.
That's a good point, aspect ratio is not a problem, but when scaling up I will provide a option for how you want me to scale it up:
"Nearest" will just stretch without filtering, it may result clearer image in best case and in pixelated results in worst case scenario
"Linear" will do a linear interpolation when scaling, it may result in smoother imager in best case and it blurrier in worst case scenario