How long can my text message be?
By default, a text will be sent by a GSM 7 bit encoding. You can use 160 characters per text. Most characters exist within this 7 bit encoding.
When using a character that doesn't match the 7 bit encoding, your device will switch the entire message into a message of 16 bit encoding (or UCS-2). When using a 16 bit encoding, the number of characters per text is limited to 70.
It will always be the phone that decides which encoding will be used. Operating systems like Symbian (Nokia) and Blackberry foresaw an option to deactivate this switch. Special characters weren't used then but replaced by similar signs that used a 7 bit encoding (e.g. à changed into a, ç into c...). Unfortunately, this option isn't available anymore on recent operating systems like Android ICS, iOS... A counter in the text application will fix this by showing the number of characters used and/or the number of texts that will be sent.