Adding Custom Font Sizes to TinyMCE editor

// Enable font size & font family selects in the editor
if ( ! function_exists( 'wpex_mce_buttons' ) ) {
	function wpex_mce_buttons( $buttons ) {
		array_unshift( $buttons, 'fontselect' ); // Add Font Select
		array_unshift( $buttons, 'fontsizeselect' ); // Add Font Size Select
		return $buttons;
	}
}
add_filter( 'mce_buttons_2', 'wpex_mce_buttons' );

https://www.wpexplorer.com/wordpress-tinymce-tweaks/