Add select field to TinyMCE to change line height

function add_style_select_button($buttons) {
    array_unshift($buttons, 'styleselect');
    return $buttons;
}
add_filter('mce_buttons_2', 'add_style_select_button');


function my_mce_before_init_insert_formats( $init_array ) {

    $style_formats = array(
        // These are the custom styles
        array(
            'title' => 'line 2',
            'block' => 'span',
            'classes' => 'line20',
            'wrapper' => true,
        ),
        array(
            'title' => 'line 3',
            'inline' => 'span',
            'classes' => 'line30',
            'wrapper' => true,
        ),
    );
    // Insert the array, JSON ENCODED, into 'style_formats'
    $init_array['style_formats'] = json_encode( $style_formats );

    return $init_array;

}
// Attach callback to 'tiny_mce_before_init'
add_filter( 'tiny_mce_before_init', 'my_mce_before_init_insert_formats' );

css

/* custom css */
.line20 {
    line-height: 2em;
}
.line30 {
    line-height: 3em;
}