custom field in the_content or get_content

I would like to use a custom field within the_content() and want to know if this can be done. I have a custom field called mask_page_height that I would like to set my iframe height to:

<iframe name="home" src="https://www.YourHome123.com/forms/155_Search100sOfProperties.asp?userid=999300" id="myiframe" width="100%" scrolling="no"  height=<?php echo $page_height; ?>>
<p>Your browser does not support iframes.</p>

</iframe>

<?php
$page_height = get_post_meta($post->ID, 'mask_page_height', true);
?>

In view source – value is not set just displays above code.

Any help would be greatly appreciated do to cross-domain and masking issues.