The maximum data size of a stream item is determined by the minimum of the following blockchain parameters (with a few hundred bytes subtracted from the first two, for wrapping).
maximum-block-size max-std-tx-size max-std-op-return-size
The current alpha (27) of MultiChain allows a chain to be configured to allow up to 8 MB of data per stream item. The next alpha, to be released shortly, allows up to 64 MB.
