Customer wants to download small size images

Garry

Hi, I have a number of Galleries (12) in one category with full size and small size images. A client would like to be able to fast view and evaluate those images for a calendar project. They will then pick a few and would like to download a small version so they can do a mock-up calendar.
What would be the best way for me to manage this request? I certainly don't want that fullsize images are becoming available as a download.
Thanks