This can be helpful when developing or adding onto a pre-built site that may have various ways an image size could have been set. I found on some sites that I was updating there were hidden image sizes that work well for various design aspects – and in keeping the loading time down on a page.
This code can be added to a development theme to see all possible image sizes on the front end:
global $_wp_additional_image_sizes; print '<pre>'; print_r( $_wp_additional_image_sizes ); print '</pre>';
Just add the above into your theme and refresh that page and you’ll get an array of all possible image sizes.