This is possible using CSS, but honestly it is not very straightforward, because there are many (dynamic) elements that need to be adjusted.
You could try adding this CSS to see what I mean:
width: 500px !important;
height: 200px !important;
I would be happy to try to assist with any tweaks you need if you provide me with a link to your site.
I do plan on dramatically improving the Overlay Windows in the future, I think being able to specify dimensions through a setting (or Shortcode option perhaps?) would be a nice addition. Feel free to post any other thoughts here.