由于主题，Wordpress图片尺寸太小

Question

My theme generate my img portfolio with this code :

<img width="222" height="440" src="#" class="attachment-portfolio" alt="#">

I found this code :

<div class="carousel-inner">
    <?php foreach($attachments as $attachment) {
        $alt = get_post_meta($attachment->ID, '_wp_attachment_image_alt', true);                    
        $image_big =  wp_get_attachment_image( $attachment->ID, 'portfolio' ); ?>
        <div class="item">
            <?php echo $image_big ?>
        </div>
<?php } ?>

I'm not good in PHP but i understand that the image size is generate from "$image_big" ?

I found also this in my function.php

add_image_size('big',9999,400);

I try to change the width and height in the add_image_size but nothing happend on the website ! It's still width="222" height="440" I would like the image to be 100% or around 800px width (if it's impossible to get % unit). What to do ?