Featured Image oversized on item description page

Hi, I′ve been playing with this theme for two days trying to integrate an e-commerce site using EDD and my digital books.
Happy so far with the options, however, I′m having issues with the Featured Image: the description page of the item displays an over-sized image. The original image is 154×200 and it′s scaled as 666×865. Looking at the style.css I found out that changing

.attachment-product-image-large{
    margin:0 20px 20px 0;
    width:100%;

to

.attachment-product-image-large{
    margin:0 20px 20px 0;
    width:30%;

it fixes my problem.

My question is if there is another workaround to fix this or any option I′m missing in the dashboard. Thanks