As mentioned here the default image sizes in WordPress are thumbnail, medium, large and full.

In the class Image_Upload_Helper the fallback size is post-thumbnail. Obviously an additional prefix post- has been used (see lines 128 and 141 in Image_Upload_Helper.php).
That’s wrong, actually. So, the prefix should be removed.

https://www.ads-software.com/extend/plugins/image-upload-helper/