How to set custom image sizes in your RSS feed
Use the "Set the size of feed images" drop-down to choose either a preset site-wide size (standard WordPress sizes, theme-created sizes), or choose "custom", and it will prompt you for your own desired sizes (width/height) in pixels. Keep in mind that it scales the image proportionally, so only width or height may take effect, depending on which one is larger from a scaled proportional size. Try out different values, set only the width or only the height, and view your feed to see how it looks.
Note that some services don't like the responsive image output that WordPress includes with images (srcset), and so display broken image links. Mailchimp's campaign preview is one that we know of. While we hope they fix this soon, for now, we've included a Disable Responsive Images checkbox to help with this; simply check the box, and it should resolve the issue.