The Feed tab in the settings page allows you to set the related posts settings for posts that appear in your WordPress site feed. This only applies if you choose to automatically display the posts in your feed via the settings under the General tab.
Number of posts to display
Enter the number of related posts that will be displayed in the feed content.
Show post excerpt
Optionally display the automatically generated excerpt for each of the posts in the list.
Location of the post thumbnail
As there is no way to style the posts in the feed, I recommend to not display thumbnails.
Thumbnail width and Thumbnail height
This only applies if you choose to display thumbnails in the previous setting.