question

In general, it's the same as the limit on the equivalent non-publisher data, but across titles (so Publisher Data would have the same limits as Title Data, User Publisher Data would have the same limits as User Data, etc., but across titles rather than per title).

That said, we are rolling our our entity storage system shortly - please have a look at the docs we'll be posting on that. The entity storage system is designed to be more flexible and efficient, so we'd encourage developers to start using it right away (and yes, entities can be at the Publisher ID level).