1) We use the photoswipe library to display that popup and it seems this JQuery library script calculates the height of the popup depending on the featured image proportions. So, I would recommend you to change the image proportion to square http://prntscr.com/pegjvy if you use squared featured product image because we are not able to edit the original js code of the photoswipe related to this moment.
2) Yes, no problem just add the following code to child theme style.css
