Wonderm00n

The WordPress default galleries always link to the full image size which can be pretty annyoing because most users upload huge pictures.
With this plugin you can set a default size for the galleries to link to. This can be thumbnail, medium, large or full.
For now this a global setting for all galleries, but on a future version I plan to let you choose a specific size for each gallery you create on your WordPress posts or pages.
It’s also possible to add arguments to the link (A) tag. This may be useful to add lightbox/fancybox script compatibility.
Based on the oikos.org.uk solution that can be found at http://oikos.org.uk/2011/09/tech-notes-using-resized-images-in-wordpress-galleries-and-lightboxes/

Wonderm00n's Gallery Link Size Changer

This plugin allows to change the way the Wordpress gallery links. You can select to…

Jason Tucker

This plugin will display the images defined using add_image_size in function.php and from installed plugins. Values are displayed in the Media Settings screen. 
You can see how images are https://codex.wordpress.org/Function_Reference/add_image_size
This plugin is based on the plugin: “Image sizes in admin dashboard!” by mansoormunib

Display Registered Image Dimensions

This plugin will display the images defined using add_image_size in function.php and from installed plugins. Values are displayed in the Media Setting …

Wonderm00n's Gallery Link Size Changer vs Display Registered Image Dimensions