Add new font sizes

// into functions.php
add_theme_support( 'editor-font-sizes', array(
  array(
    'name'      => __( 'Small', 'snippress' ),
    'shortName' => __( 'S', 'snippress' ),
    'size'      => 12,
    'slug'      => 'small'
  ),
  array(
    'name'      => __( 'Normal', 'snippress' ),
    'shortName' => __( 'M', 'snippress' ),
    'size'      => 16,
    'slug'      => 'normal'
  ),
  array(
    'name'      => __( 'Large', 'snippress' ),
    'shortName' => __( 'L', 'snippress' ),
    'size'      => 24,
    'slug'      => 'large'
  ),
) );