A C4 envelope is 9 inches wide and 12.8 inches tall, or 228 millimeters wide and 324 millimeters tall. It is designed to hold letters written on size A4 paper without bending or folding. A4 paper is slightly smaller than a C4 envelope.
The C series of paper size is used only in regards to envelope size in the International Organization for Standardization's measurements. The ISO determined that the geometric mean of the area of A and B series paper is equal to the area of C series paper with the same numerical identifier. Thus both A and B series paper documents of a given numerical identifier can fit securely in a C series envelope with the same identifier without damaging the paper.