Is there a limit on the number of characters used in an SMS?

No, you are not limited to a certain number of characters. However, each time you write more than 160 characters a new SMS will be needed to include your entire message.  For example, if you send a very long message you might end up sending 2 or more SMS.  

On Step 2 of your SMS creation, we indicate the number of characters used and the number of SMS required for sending your message.