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 http://codex.wordpress.org/Function_Reference/add_image_size

This is my first plugin, I based some of the code on the plugin: Image sizes in admin dashboard! by mansoormunib. My code can be found on GitHub.

Display Registered Image Dimensions Plugin has been submitted to the repository but for now can be downloaded here:
display-registered-image-dimensions.zip