To help our members, Flickr’s shopping cart only shows a print size if the original image meets our minimum resolution requirements for that size. We'll send the full-resolution photos to our print labs, and they'll optimize the image for their specific printer to make sure the quality is top-notch.
Wondering about DPI? You don't have to. It won't matter at all for displaying on Flickr or printing.
Keep in mind that these are minimums.