I don't know about anybody else, but for me 800px (max width of the box) is not enough and there's a lot of wasted space left on the right side.
This could be useful for HDTV/Widescreen monitors owners.

P.S.: forgot to mention that it changes width on both "Edit Entry" & "Update" pages. If you want it to take effect on only one of those pages, just change: '@-moz-document domain("www.livejournal.com")' to '@-moz-document url("http://www.livejournal.com/update.bml")' or '@-moz-document url("http://www.livejournal.com/editjournal.bml*")' accordingly. Thnx, 90d@userstyles for pointing that out.