The maximum number of characters per message is 32KiB. The maximum message size is based on the final escaped character count, including the channel name(s), effectively performing an encodeURIComponent() on the string prior to publishing. And remember that if you are using message encryption, it adds as much as 30% to the message size.
You can also check the message size before you send it instead of sending and then getting a message too large error.
If you anticipate sending messages larger than 32KiB, please read further on best practices for sending large messages and files with PubNub.
This 32KiB maximum is available to all PubNub accounts.