Open a new window in your browser (probably via the File and New
pulldown menu items). Drag one corner of the new browser window to
coincide with one corner of this red image. Drag the opposite corner
of your browser window to exactly cover the image.
You will then have a browser window precicely sized to 800 by 600 pixels. You can use that copy of the browser to check your Web pages to insure they look good at that resolution.
The same trick can be used for any resolution -- just do a File Save As on this Web page (you have my permission) and change the sizes appropriately in the source HTML.