the javascript method just provides a @font-face stack and the right urls for every browser.
you can use this stack as follows:
font-family:your-google-font-name, arial, helvetica, freesans, sans-serif;
if the javascript is disabled in the browser, the fonts will fall back to arial, if arial is not installed, to hevetiva - and so on
different heights should not be the problem, because usually you have fallback fonts, that behave and look nearly the same as the original wanted font.
with javascript, there is no problem setting it unobstrusive.
make an html className with Jasvascript (i prefer jQuery here):
(function($){
$(document).ready(function(){
$('html').addClass('withJS');
});
}(jQuery));
now you can set different styles with or without Javascript just in CSS.
to set different heights, depending on the actual font-family use (example on body tag)
var myFontFamily = $('body').css('fontFamily');
// Log to see whats the name you are working with:
console.log(myFontFamily);
if(myFontFamily == 'your-family-name-here'){
$('body').css({fontSize: '20px' });
} else {
// do something else...
}