I have read in the Vmware VMFS 5 documents, it's mentioned now VMFS maximum size will create up to 64 TB.
In my question what is the block size of VMFS 5 64 TB.
... if you are asking about the maximum virtual disk size, it is still limited to 2TB minus 512 Bytes.
see e.g. blog post and comments at http://www.yellow-bricks.com/2011/07/13/vsphere-5-0-what-has-changed-for-vmfs/
The vmdk remain with the same limit... but with a RDM you can have larger disk.
The block size of new VMFS5 datastore is fixed to 1MB, but on converter datastore it keeps the original block size.
An RDM can be >2TB as long as you use Physical mode
VMFS5 block size is 1MB on new VMFS5 datastores, upgraded datastores will retain their existing block size.
Thin VMDKs will now grow in 1MB chunks (was 2x block size), snapshot vmdks will still grow in 16MB chunks
As noted in my blog, upgraded VMFS3->VMFS5 partition will still grow VMDKs in 2x block size.