Source: wpexplorer.com
// Customize mce editor font sizes
if ( ! function_exists( 'wpex_mce_text_sizes' ) ) {
function wpex_mce_text_sizes( $initArray ){
$initArray['fontsize_formats'] = "0.25em 0.5em 0.75em 1em 1.25em 1.5em 1.75em 2em 2.25em 2.5em 2.75em 3em 3.25em 3.5em 3.75em 4em 4.25em 4.5em 4.75em 5em 5.25em 5.5em 5.75em 6em";
return $initArray;