Update spec for span batch field limit

specs/span-batches.md

@@ -144,6 +144,14 @@ Where:
 
 [EIP-1559]: https://eips.ethereum.org/EIPS/eip-1559
 
+Every field size of span batch is limited to `MAX_SPAN_BATCH_FIELD_SIZE` (currently 10,000,000 bytes,
+equal to `MAX_RLP_BYTES_PER_CHANNEL`). There can be at least single span batch per channel, and channel size is limited
+to `MAX_RLP_BYTES_PER_CHANNEL` and you may think that there is already an implicit limit. However, having an explicit
+limit for each field is helpful for several reasons. We may save computation costs by avoiding malicious input while
+decoding. For example, lets say bad batcher wrote span batch which `block_count = max.Uint64`. We may early return using
+the explicit limit, not trying to consume data until EOF is reached. We can also safely preallocate memory for decoding
+because we know the upper limit of memory usage.
+
 ## Optimization Strategies
 
 ### Truncating information and storing only necessary data