vSphere 5 moves the goalposts on LUN sizing for VMFS
vSphere 5 will totally change the way you design/size your LUN’s for VMFS. Features like VAAI (Hardware Offloaded Locking), VMFS max size increasing up to 64TB, block size 1MB only, SDRS and Storage Profiles mean the old notions of VMFS design are no longer true.
Previously both SCSI reservations and the VMFS max size of 2TB placed a ceiling on LUN size but this ceiling has been removed. Although the ceiling is gone this doesn’t mean you should create 50TB VMFS’s, I suggest the number and size of vmdk’s will influence LUN size.
Good design principles still consider the workloads being placed on the LUN’s to correctly size VMFS’s but I expect average VMFS size to increase considerably from 500GB to 2TB on vSphere 4 up to say 2TB to 10TB or even larger depending on use case.