...my hero!

IT WORKS!!! :D

is this another patch or another link/way to apply the same patch?

its a different thing.
see my following posts to that one to see what worked.

Neither of these patches solved the problem for me. The original problem is that whenever I choose any resolution above 800x600, the picture just gets smaller and the black frame around it gets bigger. It's achieving better resolution by just zooming out and not being in full screen any more.

The patch that adds a menu option with my current desktop resolution technically works - it does add the option to choose that resolution, but it doesn't make the game run in proper full screen - it just makes it smaller.

Also, none of the desktop resolutions available to me above 1280x1024 can be divided by 8, and that one was available to begin with, before applying the patch.

Does anyone have any ideas how I could just make the game run in any resolution above 800x600, without it being a small box in the middle of the screen?

Thanks in advance!

I think you've seen the main menu / opening video and expect it to be the same in-game.

Have you tried playing a map? The main menu being a bit weird as you describe is just a side effect of playing at a higher resolution. The background wallpaper they used for the menu is locked at 4:3 and uses a lower resolution. It doesn't stretch the image to full-screen as it doesn't have to.

You can easily check if black bars are rendered by the game or part of the GPU if you can move and see your mouse into the black areas around the image. If you can, its part of the game, otherwise its a setting of your GPU that 'fakes' the black bars to keep the aspect ratio intact. This works with any game really. Usually, most GPU settings are set to just stretch to full-screen, as people like their screen filled up I guess, even if it messes up the original aspect ratio.

I has always been the same for me here as well. I've never seen it as an issue, as long as it works fine when im actually playing.

For anyone trying this patch, I remember it works in a very weird way. I dug up some old chats I sent to someone after I figured it out. This is what I posted about how the patcher works:
"Uhm, ok, so this patch works in a weird way. Did you drag and drop the SC3U.exe file onto sc3k-patcher.exe? As if the latter was a folder and you wanted to move SC3U.exe into it. Its the first time I've seen a patcher work like that."

Hopefully this help for anyone that wants to try the above patcher.

Thank you for this information. This patcher worked perfectly for the GOG version I purchased earlier today.

How can someone whose native monitor resolution isn't divisible by 8 (1440x900) tell if the widescreen patch works or not?

I'm afraid I can't remember much of the game or the patch process :(

You could perhaps take a screenshot and check its resolution.
Or compare your screenshot to the original 4:3 sceenshots, e.g.
https://www.mobygames.com/game/windows/simcity-3000/screenshots/gameShotId,10461/
to see if your game screen is stretched. The wider the screen, the lesser the relative width of the sidebars: https://www.wsgf.org/dr/simcity-3000/en