I never specified a limit. Theoretically the limit is your video memory (plus all other assets). I never tested anything above 4000px wide myself, but I have heard of a map creator who was using a 11.000px wide image and it worked, so the limit should be high enough.

I tried with a 8192x8192 png that worked with the 'Even better Imager overlay" mod (https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=814102166) but it seems to crash the engine.
Could you add a feature to stretch the image in x and y separately if not in the same projection as the game map? also would be nice to have a lock feature to not accidentally move it when perfectly aligned.

Here is my overlay: https://www.dropbox.com/s/l5y3dop56y5t9ck/overlay.png?dl=0 Loading a savegame with that in the Files folder causes the game to hang during loading.

14.000x14.000 with 8GB RAM
https://www.youtube.com/watch?v=U2aOiSKlofc
with 20000x20000 crash Skylines
I think with more RAM it ist posible to load a biger Image.

I'm actually trying with a 30mb file (20kx20k pixels), the game doesn't crash but the overlay doesn't work. I get a big "?" on the overlay texture in-game.

If you want to do big maps I found A site where you can download high res png files. it is too expensive for many games uses, however there is a free trial. it lets you download poster size png images: http://www.maptive.com

引用自 Marco6158：

I'm actually trying with a 30mb file (20kx20k pixels), the game doesn't crash but the overlay doesn't work. I get a big "?" on the overlay texture in-game.

Did you found a way to fix this? I have the same issue

My current overlay has over 350 MB 11k x 11k pixels and it loads faster than a 30MB 8k x 8k. You may not set the texture quality down then it stays sharp.