Raw Match Pattern:
(<font\b[^>]*size=)(['"]?)(?![123]\b)\d+\2([^>]*>)

Raw Replace Pattern:
$1"3"$3

Note that you can have a class= attribute on <font> tags that will cause this process to fail to recognize font sizes properly.

http://www.myregextester.com/?r=232

Hey, that worked really well. Thanks a lot.

I knew Regular Expressions were powerful, but I didn't realise you could do that much with them!