A C5 envelope is a rectangular envelop that measures 162 by 229 millimeters, or 6.4 by 9.0 inches. It can hold a piece of A4 (or letter-sized) paper folded in half or a piece of unfolded A5 paper.
C Series envelope sizes range from C0 to C10. C0 is the largest size, measuring 917 by 1,297 millimeters, or 36.1 by 51.5 inches. C10 is the smallest size in the series and measures 28 by 40 millimeters, or 1.1 by 1.6 inches. A C4 envelope is an alternative to the C5. It measures 229 by 324 millimeters, or 9.0 by 12.inches. It can hold a letter-sized piece of paper without having to fold the paper in half.