As shown by the thumbnail above, in the ACP it's set to a max of 800px width. Tested it with larger images, works good.
Now, if a poster wanted the readers to see the larger version of an image is this something the poster has to code in the editor so the image can be clicked on in the forum? As it stands, the 800px image shows no clicking to do such.
A related question is about thumbnails: Thumbnails do click/redirect but I'm not sure if that's to an 800px width version on my server or elsewhere for a larger version.