Cannot use Manual Image Crop on any image sizes except thumbnail

I have three image settings in my Media Settings: http://screencast.com/t/EhwWV57WUAIS

I also defined custom images in my theme’s functions.php as per Using Custom Image Sizes, and I can use them in a post just fine: http://screencast.com/t/q8xSPyJGuXR

I want to use Manual Image Crop to crop only the medium size. However, Manual Image Crop only lets me choose the Thumbnail size. http://screencast.com/t/CkuuLNjVkLWk

And the settings for Manual Image Crop also only seem to know about the thumbnail size: http://screencast.com/t/kx7oJSgWmJ .

How can I get Manual Image Crop to let me crop the medium and other sizes for images?

https://wordpress.org/plugins/manual-image-crop/