Upgraded VMFS-5 volumes inherit and maintain the VMFS-3 block size and the size limitations that go along with the VMFS-3 block sizes. Those are listed in the kb.
The default block size for new VMFS-5 volumes is 1MB. The maximum file size, regardless of block size (new or upgraded), is 2TB - 512Bytes. This means that VMDKs greater than 2TB minus 512 bytes are not possible.