Feature Image Cropping

What is the default image size that the feature image is cropped to? Or rather, is there a certain size or pixel ratio I should be using?

Im not sure if its variant on the theme or if its part of WP

Blog is https://www.audiblechem.com . As you can see many of our images on our posts are cropped from their original size on both the thumbnail and the image at the top of the posts themselves.

Odd thing is that for images that are super massive (for instance one thats roughly 3000 pixels x 2000 pixels) the images seem to be fine and scaled nicely without cropping, but when I use one that’s maybe 500×500 it’s all cropped down to whatever was in the center of the picture without any scaling.