A maximum message size of 160 characters applies to a single SMS message. If a longer message (multi-part message) is sent, each message has a maximum of 153 characters due to seven characters being needed to facilitate re-joining of the message on delivery. For longer messages sent, the SMS is split into two or more messages that may be reassembled on delivery, so they are a single message (or, on some handsets, may be delivered as a series of separate SMS).
If your message is over 160 characters in length, every 153 characters or part thereof will be charged as an additional SMS message.
For example, if an SMS with 320 characters is sent, the SMS will be sent as 3 message parts. The first part will contain 153 characters, the second will contain 153 characters and the third part will contain 14 characters (153 + 153 + 14 = 320 characters).