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?
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,