Same problem here. I would like to delete automatically the smallest one but its so painful left right and then right-clicking all the time.
Also why cant i use keys like delete to remove the duplicated files and arrows to navigate.
I've stumbled upon the tool
Duplicate File Eraserhttp://www.f2ko.de/en/dfe.php
Its analyses files pretty quick and it can select except one and then delete the rest.
It would be useful to include such feature into xnview.