I believe the limitations are 3MB total file size and 4K total resolution (2000 x 2000 for example). (see edit below)
(I moved this out of tech support and into site feedback as this is not a Krita question)
Edit: Here is the best answer I find by searching past posts: 3MB and 9 MP max. Technical points to consider when uploading a picture in the forum gallery? - #3 by raghukamath