Pixelwelt

Image Sizes on Demand generates sizes of an image at the moment they are requested. It tries to generate the new size 
from the original image, if it exists. This means you don’t need to regnerate all your images if you add new iamge 
sizes.
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Image Sizes on Demand

Creates the new size of an image, if it is requested and does not already exist.

mansoormunib

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 <code>add_image_size()</code> is picked up automatically, alongside the built-in <code>thumbnail</code>, <code>medium</code>, <code>medium_large</code> and <code>large</code> sizes. Each row shows:
<ul>
<li>The image size name (human readable)</li>
<li>Width in pixels (— when unconstrained)</li>
<li>Height in pixels (— when unconstrained)</li>
<li>Crop mode (Cropped or Proportional)</li>
</ul>
This is handy when building a theme and you need a quick reference of the exact dimensions available to <code>the_post_thumbnail()</code> and the media library.
More detail on registering custom image sizes: https://developer.wordpress.org/reference/functions/add_image_size/

Image Sizes in Admin Dashboard

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Lists every registered image size — built-in and custom add_image_size() sizes — with width, height and crop mode in a Dashboard widget.

Image Sizes on Demand vs Image Sizes in Admin Dashboard