I can’t quantify in bytes, but a shard comes with quite a bit of baggage, it has a mapping and other data stores in the cluster state, some other bookkeeping data attached to it (where, is it primary or replica, in sync or not, ...) and each shard allocates a chunk of the HEAP for index operations. That chunks size depends on whether the shard received writes or not and ranges from 5MB to 256(?)MB. The exact maximum varies from version to version and I don’t think it’s in the ES docs.
