If it is a local datastore (created on direct attached disks), you have form a logical volume (RAID) on the hardware level in order to have the provision of expanding the datastore at a later point in time. If it is an array with LUN being presented to the hosts, you can create a datastore on one LUN for now. When you want to expand it, you can either expand the existing LUN or add a new LUN as an extent to the existing one and then expand the datastore.
Cheers,
Supreet