I see two problems with the maxMessageSize attribute of RTCSctpCapabilities:

1. An unsigned short provides insufficient space. I'm not entirely sure what the maximum message size of SCTP effectively is, maybe @tuexen can add a statement to that. However, streaming APIs should be able to handle messages of arbitrary size (although the W3C API is not a streaming API, other implementations may provide one).
2. We should clarify that 0 indicates that the other peer can handle messages of any size. This is particularly important once SCTP ndata will be deployed.

For reference, here is the SDP section relevant for WebRTC.