This worked for me:
// Customize mce editor font sizes
if ( ! function_exists( 'wpex_mce_text_sizes' ) ) {
function wpex_mce_text_sizes( $initArray ){
$initArray['fontsize_formats'] = "10px 12px 13px 14px 16px 18px 20px 22px 24px 26px 28px 30px 32px 34px 36px 38px 40px 42px 44px 46px 48px 50px";
return $initArray;
}
}
add_filter( 'tiny_mce_before_init', 'wpex_mce_text_sizes' );
Found here: http://www.wpexplorer.com/wordpress-tinymce-tweaks/
How do you move the location of the font size drop-down menu on the tinymce toolbar, though?