Mark is right.
There is no direct way we can calculate exact memory of persisted object.
Assuming that you just want to care about the storage of your application alone. You can fetch the persistence memory accessing the key and fetching the vector from that memory.
After than
Either you can count vector's size. and easily show number of objects stored in memory.
Or
Fetch every object from vector. Count the bytes and convert them into mb and show them, but afaik it will not be convenient way, neither we can get perfact data.
