If I have it right, the Moodle TinyMCE is not the full version, but a trimmed one that provides the immediately required functionality. AFAIK, you can reset text sizes by finding the fontsizeselect button code and replace the existing code with this.
tinymce.init({
selector: 'textarea',
toolbar: 'fontsizeselect',
fontsize_formats: "8px 9px 10px 11px 12px 13px 14px 15px 18px 24px 36px 72px"
});
Check to make sure the code says what it says above, just add in the additional sizes. Where it is in the code I have no idea, Peter has done an excellent job on obscuring everything..