The maximum size of the data field in a stream item is the smallest of:
- The max-std-op-return-size value.
- The max-std-tx-size value, minus a few hundred bytes.
- The maximum-block-size value, minus a few more hundred bytes.
These can all be viewed by called getblockchainparams and adjusted in a new chain's parameters, before the first block has been mined:
http://www.multichain.com/developers/blockchain-parameters/