We allow a maximum of 160 characters for your first message body. This counts as 1 message credit per recipient. Subsequent message bodies (that is 2nd to 5th) can only have a maximum of 150 characters and the message cost is calculated per recipient accordingly.
Also note that the “enter” key and other characters copied from Word or from your browser may count as more than one character so make sure to check your total message cost on the verification page before you send. Please see more on Pricing here