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.
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..