By default WordPress allows you to customize thumbnail. medium and large image size. In this lesson you will learn about enabling all other image size selection options on media size dropdown, which you use while inserting images inside content with ADD MEDIA button.
WP Image Size Selection is a WordPress plugin that automatically adds any available image sizes to the media size selection drop down on your website.
Start by installing ‘WP Image Size Selection’ plugin in WordPress
Upon activation, visit your admin area dashboard Posts -> Add New screen or open any old post or post type to edit.
Use ADD MEDIA button to upload and insert new image, and you will be noticing the plugin has added more size options you can make use of.