There's an old entry in the WordPress FAQs that gives information about how wide the images can be on each theme: http://faq.wordpress.com/2006/09/25/how-big-can-my-images-be/
Unfortunately, that entry is outdated. Information about many new themes are not provided, and some old listed themes are no longer available.
I think it would be great if someone updated that entry and placed it in the new Support section. That information is valuable because some people, like myself, want to post images that are large, but not so large that they break the display or cause the image to become resized by the browser.
There's currently no link for "Images" listed in the Media section in Support. Perhaps this would be a great way to start it off.