The Media settings page allows you to set the default size for your images. If you find the thumbnails too small or too large, then return here to change the default size. To begin with, you can leave these settings as they are.
The auto-embeds option is useful. It allows you to simply copy the web address (URL) for a video on some services, such as YouTube and Vimeo and paste it into your post or page. It will then automatically display the video on your website. You can read more about this feature on the WordPress.org website.
Read more on WordPress.com