Please check whether the datastore was created with the VMFS3 or VMFS5 filesystem. Before VMFS5 which was introduced with ESXi 5.0, you needed to specify a block size (1, 2 4 or 8MB) while creating a datastore to allow files/virtual disks with sizes of 256, 512, 1024 or ~2048GB (see http://kb.vmware.com/kb/1003565).
VMFS5 uses a unified block size of 1 MB for all supported sizes of files/virtual disks (currently up to ~2TB)
André