vmkfstools --createfs vmfs3 --blocksize 8M vmhbaA:T:L:P
1. Move any virtual machines off to other storage. (NFS, iSCSI, or somewhere else)
2. Logon to the console as root
3. Execute the vmkfstools command, this will reformat the partition with the block size specified (2M, 4M, 8M) and without a volume label.
4. Logon to the VI Client
5. Select the host, configuration tab, storage, and refresh.
6. Find the local storage with the UID number, and rename it to a logical name.
7. Move virtual machines back to local disk.
Additional Ref: how-to-change-the-blocksize-of-a-vmfs-filesystem
For ESX 4.x take a look at this KB from VMWare. KB Article: 1012683