Source

File: wp-admin/includes/class-wp-debug-data.php

public static function get_database_size() {
		global $wpdb;
		$size = 0;
		$rows = $wpdb->get_results( 'SHOW TABLE STATUS', ARRAY_A );

		if ( $wpdb->num_rows > 0 ) {
			foreach ( $rows as $row ) {
				$size += $row['Data_length'] + $row['Index_length'];
			}
		}

		return (int) $size;
	}