It's not an element, it contains the value of the style property if you wanted to reference them in LESS. For example:
Less:
html
{
font-size: @xf-fontSizeNormal;
}
Saying that, for what you're looking to accomplish it may be better to use rem font sizes in the style properties and then set your base font size on the html element accordingly (using media queries to make the base size larger on mobile). I haven't tested this approach though so YMMV.
Then using rem units in the style properties. These units are proportional (multipliers) to the sizes set above:
Normal font size: 1rem
Small font size: .875rem
Smaller font size: .75rem
Smallest font size: .625rem
Large font size: 1.125rem
Larger font size: 1.25rem
Largest font size: 1.375rem
You'd need to adjust the base values and style properties to your liking, they don't quite correspond with what you've listed above. I think this is your best bet for accomplishing this though.