Are you better off sending a large letter or a small parcel? Find out with our handy 2024 guide to Royal Mail parcel & letter sizes. Updated July 2024 Postal Boxes 15 X MAXIMUM SIZE ROYAL MAIL SMALL PARCEL PACKET POSTAL BOX Parcel Boxes Dimensions: 420 x 340 x 154mm. Specifically designed for Royal Mail Small Parcel Shipping. In Stock. Available for next day delivery or standard 2-3 day... MANUFACTURED TO HAVE MAXIMUM EXTERNAL DIMENSIONS AS ALLOWED UNDER THE ROYAL MAIL 'SMALL PARCEL' SYSTEM EFFECTIVE 1 APRIL 2013 W E Roberts Packing Supplies Ltd Royal Mail Small Parcel Box, Shipping Boxes (44.9x34.9x7.9cm) Postal Boxes, Cardboard Boxes for Posting, Gifts (Pack of 10 Internal Dimensions: 442x342x66mm (17., 5x13., 5x2., 6)