Featured Image Post-Thumb Dimensions

Can’t find in your documentation where you set the dimensions/cropping of the featured images. I’m not a fan of the squared featured images on posts or in category/archives, but otherwise love your theme. I can add a post-thumb class in my style.css to overwrite it a bit, but the quality of the image is greatly diminished because it’s being altered somewhere else first….I just can’t seem to find WHERE or HOW.