Chris (from our team) has been working to get some application-specific data regarding serialization packages. So far he has: AttestationRecord, Block, ShardAndCommittee, CrosslinkRecord, ValidatorRecord and CrystallizedState serialized in Cap’n’Proto, Protobuf and FlatBuffers.
You can find the code in this repository: https://github.com/sigp/serialization_sandbox
So far we have a test structure and some information regarding size for a few different packages. You can see the “roadmap” in the repository readme.
You can run the code yourself, or here’s a printout of what we have so far: https://notes.ethereum.org/s/BkM5bqsIQ