Completes the asynchronous operation started in BeginGetSize.
Size of file in bytes.
Before calling BeginGetSize, you need to create a callback method that implements the AsyncCallback delegate. This callback method executes in a separate thread and is called by the system after BeginGetSize returns. The callback method must accept the IAsyncResult returned by the BeginGetSize method as a parameter. This parameter may then be passed to the EndGetSize method to complete the operation.