This might be slightly basic. Sorry for that but I was wondering if it is possible that the second choice font we specify in HTML (using font/font-family) can be of a different font-size.
Eg. Use Lucida Grande, size 13 or Arial, size 14 if that wasn't found
This related question: http://stackoverflow.com/questions/1271477/changing-body-font-size-based-on-font-family-with-jquery cites using this javascript font detection library: http://www.lalit.org/lab/javascript-css-font-detect in combination with jQuery (Javascript library) to achieve what you're looking for.