Which themes 800 px across

I am looking to find out what themes are set so you can easily change the width? I would like to have a theme less than 800 px across - not. I currently have nelo on a lot of my site but not sure how to set the site so that it would be 800 px across. The reason - is so that i can embed the site on a FB tab. any thoughts would be great. thanks.
Bill