Arvados

from arvados/sdk/python/arvados/commands/put.py

def _collection_size(self, collection):
        """ 
        Recursively get the total size of the collection
        """ 
        size = 0
        for item in listvalues(collection):
            if isinstance(item, arvados.collection.Collection) or isinstance(item, arvados.collection.Subcollection):
                size += self._collection_size(item)
            else:
                size += item.size()
        return size

This returns the collection size, there should be a method in the Collection class that does the same thing.