The message size limit is 10KB. The size of the message is calculated based on the number of bytes in the HTTP POST body. For more information see the Server Publishing docs.
The processing required to handle large messages can degrade the quality of service experienced by other Pusher customers so we have imposed a message size limit. Our focus is to enable distribution of small messages as fast as possible so to ensure that we can quickly parse and distribute messages we have imposed the 10KB message limit.
If you are interested in sending larger messages we can set up a dedicated cluster. If you are interested in provisioning a dedicated cluster as part of an enterprise package please contact Pusher sales.
One approach is to simply compress the messages in some way, removing as much whitespace and unnecessary characters as possible (clipping words, using single letter keys instead of full words, etc). Another option is to split out the messages into multiple messages.
You can also store the element on the server and notify the other users using Pusher that you can then retrieve the element from the server.
The approaches you can take depend entirely on the type of data you're sending and the way it's formatted.