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You can allow users to choose the theme with the theme selection block.
Just call one theme 800x600 and another 1024x768.
Or fancy way, have a gateway page before your XOOPS CMS, and use Javascript to set a cookie with the screen size. Then in the theme.html, put some php to read the cookie and if its either screen size, use that theme. And if its neither, use 800by600 theme.