How to add custom font sizes to WordPress Text Editor

In your functions.php place the following code

if ( ! function_exists( 'tiny_mce_before_init' ) ) {
   function tiny_mce_before_init( $init ) {
      $init['fontsize_formats'] = '8px 10px 12px 14px 16px 18px 19px 20px 22px 24px 26px 28px 30px 32px 48px 50px 52px';

      return $init;
   }
}
add_filter( 'tiny_mce_before_init', 'tiny_mce_before_init' );