do you mean NetGroup.post()? a post is limited to 16KB for propagation beyond a first hop. and the posting must be small enough to transfer between peers in less than 5 seconds or it will not propagate.
if you mean NetStream.send() in a multicast, then the message length is limited to 10MB, but it must transfer in less than the multicast window duration.
if you mean NetStream.send() in 1:1 (DIRECT_CONNECTIONS) P2P, then the message length is limited to 10MB.
also i noticed in your subject you say "(RTMPT)". i assume you don't mean that, but RTMFP. Cirrus (codename) does not use RTMPT (RTMP Tunneled over HTTP), which is a client-server-only mode.