I read this thread about why the plugin only uses medium size images: http://wordpress.org/support/topic/253482?replies=7
Basically saying that you'd need to do too many API calls to check each image to see if a larger size exists since not everyone has larger images and/or premium accounts.
That's fair enough. However... I *DO* have large images for every image. Why not add this feature so it's an option for those who *DO* have large images? Why punish everyone? LOL
The plugin is simply amazing. Head and shoulders above all the other Flickr plugins I looked at. I'm just not loving the small images. The beauty of a "lightbox" is that it can fill your entire screen with a big beautiful image. Many of them will even scale the image down for those who are not using a 27" monitor. LOL.