How to change the size/appearance of the featured image in a post

Hi! I am new to your theme Full Frame (and love it!), so maybe there is already an answer, but I didn‘t find it. When I use a featured image function this image will also occur in my post. But unfortunately it will be cropped. How can I change the appearance that it shows without being cropped? As a photographer most of my images have an aspect ratio of 3:2 Thanks a lot in advance Thorge