A C5 envelope, which is designed to fit an A5 paper, measures 6.4 inches by 9 inches. A5 is a paper size, not an envelope size.
The ISO organization, which is an abbreviation for the Greek word for equal, sets internationally recognized paper sizes. The United States and Canada are the only countries that do not follow these standards.
There are three series of sizes: the A and B series define paper sizes and the C series sets envelope sizes. Numbers in each series correspond. For example, A4 paper fits in C4 envelopes.
A5 paper measures 8.27 inches by 5.83 inches. A4 is the most commonly used paper size internationally, similar to the standard 8-by-10-inch paper used in the U.S.