500 KB for a file size is more than enough for most questions. The Gimp .xcf allowance is a bit larger at 1 MB to provide for questions about layers / selections / guides etc. All about bandwidth. If you have a slow internet nothing worse than waiting for some monster image to load.
12000 x 12000 is a large size, but it should work, I made one up and this took 4 minutes 20 seconds to render. Linux / Gimp 3.2.4 (appimage)
It is going to depend your computer. Try Windows -> Dockable Dialogues -> Dashboard to show the state while rendering.
Lots of small selections, it might help if you turn the "crawling ants" off View -> Show Selection ...and as you say, might be your image(s)
12000 x 12000 is a large size, but it should work, I made one up and this took 4 minutes 20 seconds to render. Linux / Gimp 3.2.4 (appimage)
It is going to depend your computer. Try Windows -> Dockable Dialogues -> Dashboard to show the state while rendering.
Lots of small selections, it might help if you turn the "crawling ants" off View -> Show Selection ...and as you say, might be your image(s)