I’ve seen this image before…very beautiful image.

If I’m understanding you right your question is trivial.

1)- PS stops turning an image 72 dpi when you tell it to. You are in total control. No one says you can’t output an image as large as you want. But if you’re using it for the web, be ready to accept long download times and its consequences.

2)-Why would you want to find the largest 72 dpi when for many years the web has been turning more towards 96 dpi and Flash vector?