General description Royal Mail Large Letter PIP Cardboard Postal Boxes are high-quality, UK-manufactured corrugated boxes designed for the Royal Mail 'large letter' postal system. These brown and white single wall die cut boxes are easy to assemble, featuring fold-over ends and "earlock" lids. Technical specifications C5 Size - Internal: 222 x 160 x 20 mm C5 Size - External: 228 x 164 x 22 mm C6 Size - Internal: 160 x 110 x 20 mm C6 Size - External: 166 x 114 x 22 mm C4 Size - Internal: 332 x 245 x 22 mm C4 Size - External: 332 x 245 x 24 mm