volume/lun size

There's no single, simple answer to that question.   How large are your VMDKs going to need to be?   How many VMs total?  How active I/O wise are these VMs?   ESXi v5 with VMFS v5 removes the 2TB LUN size limit and is more space efficient than VMFS-v3.  

In general terms, more, smaller volumes are better than one MEGA volume.  Keeping the number of IO active VMs down on each volume is also a good idea.  

-don

This will have to be assessed on a per store basis.   The use of the store will decide.