Fit Iframe to an image, in a modal

[admin]

I have tried many option but nothing works.
On <a href="http://www.skuar.com" rel="nofollow noreferrer">my website</a> clicking on any post open an iframe in a modal box (using <a href="http://noelboss.github.io/featherlight/" rel="nofollow noreferrer">Featherlight.js</a>) with the wordpress single.php

The problem is I can't fit the modal box to the image inside, I just decided to had sizes into the html code so far.

<div id="post">
<a href="#" data-featherlight="#featherlight-<?php echo get_the_ID(); ?>"> 

        <?php
        if ( has_post_thumbnail() ) {
        the_post_thumbnail('post-thumbnails');
        }
        ?>  

    </a>

<div class="myframe">
<iframe class="lightbox" src="<?php the_permalink(); ?>" width="400" height="600" id="featherlight-<?php echo get_the_ID(); ?>" style="border:none;" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe>
</div>

</div>

Do you reckon it's possible using javascript or jquery?
I only know html, css and a bit of php but if I have to...

Thanks!