C6 is an internationally agreed standard for paper of size 114mm x 162mm ( 4.5x6.4ins). For the fullest details you are advised to consult ISO216, which is available, for a fee, via the WWW. I hope that the information below will meet your need without any further trouble and expense.
ISO216 'A' size is the most commony met ~ then the 'B' ~ 'RA' ~ 'SRA' ~ sizes. The ISO 'C' range of sizes is designed for envelopes.
A sheet of A4 paper will comfortably fit into a C4 envelope ~ and so ~
A5 into C5
A6 into C6
~ and so forth. There are others in the C range, but those three are most commonly met.
Royal Mail, and I suspect many other countries, base their size limits on the ISO range. In the UK a C5 envelope containing several sheets of A5 ~ or folded A4 ~ paper will be charged as a 'Letter' (up to a certain weight limit, currently100grams for inland mail). A C4 envelope, which can contain a substantially thick pile of papers ~ up to an overall 25mm ~ about an inch ~ is charged as a 'Large Letter'.
Anything larger ~ or heavier ~ than the proscribed limit travels as a 'Small Parcel', or 'Large Parcel'.
I believe ~ and welcome confirmation ~ that some countries are not prepared to handle envelopes smaller than C6. Judging by the prevalence of various letters arriving in my mailbox I have a feeling that C5 is nowadays much preferred. Probably because the bulk automatic sorting machinery is attuned to that size ~ and that an A4 sheet only needs one fold to fit into a C5 envelope.