Hardware-wise, IBM i can scale up from basically a thick pizza box to something relatively monstrous in size. The architecture and OS are still relatively advanced, too, even by today's standards. Those of us who know its history sometimes think of it as "IBM: The Next Generation", since it was based at least in part on the Future Systems project, a replacement computer architecture that never quite came to complete fruition. IBM has never marketed it as a mainframe-class system, though.