These layouts are just to show how awesome and easy the layout editor in Builder makes building sites. I was able to create these layouts in less than a minute just by adding navigation, HTML modules, widget bar modules, and content with sidebars. Making layouts in iThemes Builder is crazy easy.
Check out this 600px wide layout (notice it’s just it’s own page in the same theme with a different layout that I made in 30 seconds).
You can make your layouts and widget areas any custom widths you want, but the 600px and 780px layouts are the preset selections in the layout editor.