100 – 13″ x 18″ Stayflats Plus Kraft Mailers. The 6PSK is an ideal size to hold 12″ x 18″ sheet prints. This very versatile size will also hold presentation folders, books or posters. 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.