Adjust Pop-up frame size and colour

Hi, I am currently testing the usage of pop-up functionality in order to offer an e-book subscription pop-up window on our start page https://kompetenz-check.ch, poping up after four seconds.
We so far managed to implement it along with some support tickets, thus using the following code on the child functions.php – however, the code does not display the forms full size so that the user will have to scroll down on the window. Also we are looking for a solution to have the background colour of the window changed in line with the start page colour, e.g. #6786a1. For your reference, you will find in the first footer column two E-Book related links (called “Bestellformular”, one was composed with an iframe tag, the other with a prettyPhoto tag. The latter one displays the form in its full size.

Following is the code:
function add_custom_script(){
?>
<script>
(function($){
$(window).load(function() {
setTimeout(function(){ $(‘.popup_trigger’).trigger(‘click’) }, 4000);
});
})(jQuery);
</script>
<?php
}
add_action(‘wp_footer’, ‘add_custom_script’);