In addition to what the above posters have mentioned, I would throw in the notion that SVmotion makes that decision an easier one to live with than it was before, where if you ran up against the max file size on a LUN it was more of a hassle to clear off the LUN (cold migrations with corresponding downtime) to reformat it if necessary. So, if your infrastructure is such that you can make use of SVmotion, then make the best educated guess of your largest potential .vmdk file and pick your block size accordingly. I tend to go with larger block sizes as I've already been burned with a VM that had to have a larger file than my VMFS volume would accomodate, and I have some room to spare on my LUNs. However, if you're more space constrained or have mostly small VMs, then go for a smaller block size. Either way, SVmotion is your friend when the unimaginable happens! 
