The following snippet will print out a list of the available thumbnail (featured image) sizes available in your current theme. Simple past the code below into a theme page and that specific page will print out the list. This is handy if you are trying to figure out the different image dimension options available to you in a theme.
<?php global $_wp_additional_image_sizes;
print_r( $_wp_additional_image_sizes );
*Important: Due to a conflict displaying the code I had to remove the pre tag. Please replace the two blocks where it says “zzz” with “pre”.