Image Sizes in Admin Dashboard adds a read-only widget to the WordPress admin Dashboard that lists all currently registered image sizes.

Any size you register in your theme or plugin with add_image_size() is picked up automatically, alongside the built-in thumbnail, medium, medium_large and large sizes. Each row shows:

The image size name (human readable)
Width in pixels (— when unconstrained)
Height in pixels (— when unconstrained)
Crop mode (Cropped or Proportional)

This is handy when building a theme and you need a quick reference of the exact dimensions available to the_post_thumbnail() and the media library.

More detail on registering custom image sizes: https://developer.wordpress.org/reference/functions/add_image_size/