The Resize Settings of EWWW IO put you back in control of the WordPress resizing process. It also allows you to limit the size of images uploaded to your site, so that your users don't use up all your storage space.
Resize Media Images
Next up, is the ability to limit the size of images uploaded to your site. This first set of dimensions is for images uploaded using Add New Media and images inserted via the post and page editors. At this point, anything else is considered an "indirect upload". Resizing will happen after all the thumbnails & various resizes are generated, so that you have the best quality possible, instead of making resizes from a resized image.
Resize Other Images
The other set of dimensions will affect any other images which get uploaded indirectly to the media library. This is often the case for theme images like logos, backgrounds, banners, sliders, and so on. Currently, images outside of the media library are never resized, although that feature is on the roadmap for future development.
Resize Existing Images
If you have a lot of images already and want those to be resized too, you can check this next box to make sure the bulk optimizer resizes them. When you run a bulk process, it will use the "other" dimensions for resizing images.
A feature that can help save time (and resources) is the ability to disable the resizes that are created by WordPress. Some sites have as many as 20 resizes listed here, so it can be beneficial to look through these and see if you need these to be created or optimized. With WordPress 4.7+ this also applies to PDF preview images. You can disable all PDF previews by disabling the full-size preview image, since that image is used to build all the smaller thumbnails.