While having a discussion about the future of Keccak in our library, we came to the discussion of how to properly assign a "block size" to Keccak (or more generally any sponge-based hash if possible), especially for use with HMAC (mostly out of API compatibility reasons).  

The block size itself is a phenomen from the days when Merkle-Damgard hashes were the hash-functions of choice (such as MD5, SHA-1, SHA-2) and the HMAC security proof relies on the fact that the key is padded to at least a full block size and thus the block size requirement can't be eliminated from the HMAC interface.

**So what's the cryptographic standard / recommended practice to assign a block size to Keccak (or a sponge-based hash)?**