WordPress - Get a first image attachment in post

functions.php

<?php
function odin_get_first_image() {
    $attachment = get_children(
        array(
            'post_parent'    => get_the_ID(),
            'post_type'      => 'attachment',
            'post_mime_type' => 'image',
            'order'          => 'DESC',
            'numberposts'    => 1,
        )
    );

    if ( ! is_array( $attachment ) || empty( $attachment ) ) {
        return;
    }

    $attachment = current( $attachment );
    echo wp_get_attachment_url( $attachment->ID,'full');
}

Hello. wp_get_attachment_url() don`t have second parametr 'size'