Scheduled Messages (SMS) (All subscribers, or new subscribers) - 160 Characters. But, within those characters make sure to follow the legal guidelines for text message marketing from the CTIA and TCPA.
Scheduled Messages (MMS) (All subscribers, or new subscribers) - 500 Characters plus an image or video if desired.
Opt-In Messages (Opt-In Message, Second Time Opt-In Message) - 160 Characters. This must include all text that the CTIA requires disclosures at the bottom of each opt-in message reading something along the lines of "Text STOP to end or HELP for help. Msg&Data Rates May Apply. Message frequency may vary."
- Double Opt-In Messages (Request for Information, Fail (Retry), No Response) - 160 Characters.
- System Responses (HELP Response, STOP Response, REPLY Response, Already Subscribed) - 160 Characters.
- All autoresponders allow 160 characters.
For messages sent to Canadian wireless carriers you get 136 characters, or 113 when sending a mass text message with the default "To opt-out reply STOP" message at the bottom.