Open Question
The shape serializer design defines the size parameter as a required integer value which this PR implements. I'm wondering if this parameter should be considered optional since it only seems applicable to protocols like CBOR which are serialized more efficiently with definite sizes.

I'm wondering if this parameter should be considered optional since it only seems applicable to protocols like CBOR which are serialized more efficiently with definite sizes.

Callers to these methods generally don't know what concrete implementation they're calling out to, so they always have to set it. len is static time generally so it shouldn't be a performance issue