Get the size of all options which are set as `autoload_size` in the WordPress `wp_options` ($wpdb->options) table.

get-autoload-options-size.php

<?php
/**
 * Get Autoload Options Size
 *
 * Get the size of all options which are set as `autoload_size` in the WordPress `wp_options` ($wpdb->options) table.
 * 
 * @todo Change the `prefix_` and with your own unique prefix.
 * 
 * @since 1.0.0
 */
if( ! function_exists( 'prefix_get_autoload_options_size' ) ) :
	function prefix_get_autoload_options_size() {
		global $wpdb;

		$size = $wpdb->get_col( "SELECT SUM(LENGTH(`option_value`)) as `autoload_size` FROM {$wpdb->options} WHERE `autoload`='yes'" );

    var_dump( $size );
		var_dump( size_format( $size[0] ) );

		// Output (Something like below::
		// 
		// array (size=1) 0 => string '454502' (length=6)
		// string '444 KB' (length=6)
		wp_die();
   	}
   add_action( 'wp_head', 'prefix_get_autoload_options_size' );
endif;