Specific Thumbnail Size for Custom Content Type

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It seems to me the quickest way to do that is:

  1. Add your specific thumbnail size to functions.php. Here’s the code to do it.
  2. Find the specific template file in your theme that displays your custom content type, then modify it to display your recently added thumbnail size. Use the following code, enter the specific thumbnail id you added in point 1 as the parameter.
  3. Additionally, you can use the AJAX Thumbnail Rebuild plugin so WordPress create thumbnails in your specific size. This is especially useful if you already have a running site with plenty of thumbnails in place.