100 – 11″ x 13.5″ Stayflats Plus Kraft Mailers. Attention collectors! Here’s an ideal size for you to mail collectibles. The 3 PSK was originally created for larger 10″ x 13″ photos, but this very versatile size will also hold presentation folders, books, catalogs and magazines. It’s made from durable .028″ Tan Chip paperboard; it will stand up to the rigors of the mail and protect the corners of your books from damage. It’s also environmentally friendly and 100% Recyclable.