The Twilio Programmable Messaging platform has a limit of 1600 characters in a single incoming or outgoing message request. This limit applies to all Programmable Messaging channels, including SMS, MMS, WhatsApp, Facebook Messenger, Line, etc.
Outgoing message requests that exceed 1600 characters will fail with Error 21617.
Incoming messages to Twilio that exceed 1600 characters will be truncated to 1600 characters, and will be marked as failed with Error 21617.
Additional Considerations for Long SMS Messages
Although Twilio supports large message payloads for all Messaging channels including SMS, it is important to understand that some wireless carriers and devices may behave differently than others when receiving long SMS messages. In addition, the length of an SMS, and which characters are present will affect its price, because of a concept called segmentation.
For more details about SMS segmentation and considerations when sending long SMS messages, please see Does Twilio support concatenated SMS messages or messages over 160 characters?