If you are sending an sms within the standard GSM 0338 character set, it will be 160 characters per sms, however if you include characters outside of this character set, the sms size will be reduced to 70 characters.

Blast SMS does allow you to send more than a single sms, this is called concatination and will require several sms characters in order to "join" the whole sms.

SMS Character Table:

Due to handset limitations, we do limit the character amount to 6 sms's in total.