encyclo - 7:01 pm on Oct 13, 2005 (gmt 0)

You won't need a full extension for this - you can do it with a simple bookmarklet. Here's the script:

javascript:void(window.resizeTo(800,600))

To add the bookmark, create a link with the above in the href, then view the page and drag the link to your bookmarks. Select the bookmarked link and your screen will resize to 800x600.