Media Image settings are subjective and depends on what you decide. The media settings in WordPress are available if you want to have cropped images to the sizes you set. When you first install WordPress and go to Settings >> Media, you will see predefined dimensions already entered:
What happens when you set these dimensions, is that every image you upload, whether for a page, post, featured image, gallery, etc., if the image is large enough, you will end up with a thumbnail for every size you see, plus the original. Over time, this can fill up your host server (and at times, very quickly). Of course, there are plugins that will let you clean out thumbnails that are not used (a topic for a future article).
Our Themes Have Our Own Image Sizes
Most themes have their own media sizes coded directly into the theme. We do this so that it helps keep the front of your site looking consistent with featured images. Each theme we release will have their own dimensions set, but what this tutorial will do is give you a quick intro about our themes.
You still have the choice of adding your own media sizes, but our themes have their own. The bonus you get is that you can choose to enable the auto-cropping of images when you upload them. Our thumbnail cropping settings let you pick and choose which ones to use. Most of the time, the cropped featured images are for the blog style you choose.
The thumbnail choice of settings will vary from theme-to-theme but the option will be in most, if not all themes.
Go to Appearance >> Customize >> Theme Options >> Thumbnail Settings
Select the ones you want to enable. Generally you just select the one for the blog style you plan to use. Then click on “Save & Publish“.
NOTE: Cropping only works if the images you are uploading are larger than the cropping size. Your theme setup tutorials will have these dimensions listed for you as they change from theme-to-theme.