Compress & Resize Images

Hi, I've smushed all my images with WP Smush, but it still shows a large number of images that need to be "compressed and resized." I saw that you suggest doing this in the media library, to scale the images to the size of where they are used in the website.

Is there a way to tell what size I need to resize them to? Right now, I am blindly guessing and resizing images.