The question is a two part question:
What is the message size limitation of stellar consensus protocol?
Also how does the message size affect the TPS of stellar?
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SCP doesn't have any theoretical limits, since really all you need to agree on is a SHA-256 hash of an arbitrary-sized set of transactions. However, the Stellar validators vote to set a maximum number of transactions per block, which is currently at 50 transactions/block. To see this value, you can query horizon for the latest block header:
$ curl 'https://horizon.stellar.org/ledgers?limit=1&order=desc' ... "max_tx_set_size": 50,