Message Length
A single text message is a maximum of 160 characters in length, including spaces. If your email is longer than this your message will be sent as 2 or more texts that are recombined on the recipient’s mobile phone as one message. You are charged for each individual text.
You can specify the maximum number of texts to be used to send the message. For example to restrict it to 1 text put restrictSize=1; in the subject field (including the semi-colon). With this in the subject field if the email message is greater than 160 characters in length only the first 160 characters will be sent in 1 text message.