The size C6 is in International Standard (ISO) of 114mm x 162mm (4.49 inches x 6.38 inches). The other sizes in the ISO series, and an explanation, is available here.
A C6 envelope will take several sheets of A4 paper (folded twice) or of A5 paper (folded once) or of A6 paper or card.
C6 envelopes can have a variety of flaps: self seal; gummed; diagonal; straight. They are normally of wallet style, but pocket style is also available (but usually at extra cost: it is not a popular style.
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