Long Messages
English Messages
You can send upto 160 characters in an English language message. But some characters like the Euro symbol take two characters. So it is wiser to keep the length lower than maximum.
Unicode Messages
You can send messages in Arabic and several other languages. These are sent as Unicode characters which can express all the world's languages. But each character consumes two or more bytes. Thus the maximum length of a single message comes down to 70 characters.
Multipart Messages
You can send longer messages and the system automatically splits messages into multiple parts. The parts are reassembled by the receiving phone and shown as one message. Some characters are reserved with each part to tell the receiving phone
- Message ID to which this part belongs
- Total Number of Parts
- Part No
As a result, the number of characters is lowered from 160 to 153 characters per part.
For non-English, the length comes down from 70 characters to 63 characters per message part
Further Reading
There are lots of resources on this subject on the Internet. The following are some non-technical resources which explain multipart messages in a simple language