Pretty Photo in the Portfolio – the video size is too small for mobile version

Hey there.

I am using the Pretty photo Portfolio page to show the video gallery of the blog. Using a custom field I put the Youtube link in the Pretty Photo box. The problem is that on mobile screen the size of pretty photo is too small, and I cannot figure out how to change it in the code. Here is the code I use:

 add_action( 'genesis_entry_header', 'epik_prettyPhoto_image' );
function epik_prettyPhoto_image() {
    $yout = get_field ( 'youtube_url' );
    $img = genesis_get_image( array( 'format' => 'html', 'size' => 'portfolio-thumbnail', 'attr' => array( 'class' => 'alignnone post-image' ) ) );
    printf( '<a href="%s" rel="prettyPhoto[gallery1]" title="%s">%s</a>', $yout, the_title_attribute('echo=0'), $img );
} 

Thank you!