The current maximum is the largest per platter drive times 4. They just came out with 667GB per platter drives, so 667GB x 4platters = 2.67TB. I thought I heard 3TB drives were coming out soon so I'm probably missing something somewhere. As we get bigger and bigger platters, we get bigger and bigger drives.
As for the largest possible drives, I'm not sure. Are we using 48Bit access now? 2^48? Many 100s of TBs.