Rich, that doesn't really jive with actual reality, at least IMO. It used to be that uploading a large graphic that it could be expanded to see detail, that doesn't work nearly as well any more. If you look at my test post above, I uploaded a large image, here's the resolution that I uploaded.
I selected "insert all graphics once the image was processed, and it did indeed insert it.
When I click on the resultant image, it does enlarge... just a little bit. It displays as a 999 pixel wide image, a far cry from the 4K pixel wide image I uploaded. Here's what I get when I check the properties of the expanded image, and it's indeed about half the width of my 1080P screen.
I guess the real question, is there a maximum size to be displayed on the forum? It "appears" to be 999 pixels wide.