The Body Font Size dropdown allows you to set the default font size for your template. Options in this dropdown are Small (12px), Medium (13px) and Large (14px).

Note that this will exclude elements where the font size is set via the templates CSS for example menu items, article titles, module titles etc.

If you wish to use a font size not available within this dropdown, you can also set the body font size via the Custom CSS field using something like the following...

body {
    font-size: 15px !important;
}

Where 15px is your new body font size.