SMS that exceed 160 characters are considered to be Long SMS.

When you exceed your SMS character limit, you will be charged for each additional message 

e.g. if your SMS is 2 messages long and your SMS rate is 7p / 7c, then you will be charged 14p / 14c per long sms. 

The system will always alert you to this (see below):

If you would like to know if there are Long SMS being sent from your account, you can double check your SMS settings with the help of this article: What SMS are being sent from my account? 

If you would like to send a Long SMS, this article will show you how to do this: How to send a Long SMS