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.
This feature is a tool for site admins to find images on their site that are more than 150% the dimensions that are being displayed within the page. The feature is only visible for admin users, and will highlight any over-sized images with a blue/teal and black border. You can safely leave it enabled to help you track down images that need resizing, and it will have no impact on what your visitors see.
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 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.