Prior to vSphere 5.5, the maximum virtual disk size is 2TB minus 512 Bytes. If you need to create snapshots the maximum virtual disk size 2,032GB due to the metadata overhead in the snapshot file. With vSphere 5.5 and VM's running on the latest HW version, virtual disks can have a size of up to 62TB on a VMFS5 datastore.
There's basically no need to leave a large amount of free disk space on a datastore. However, in case you want/need to create snapshots, remember that - as of vSphere 5.0 - snapshots are by default created in the same datastore as their parent virtual disks. In addition to this, vSphere may store some data on the datastore as well, e.g. for HA datastore heartbeats. So I'd suggest you leave some free disk space (a few GB should be ok).
As a side note. if you are going to max out the datastore capacity, you may want to re-configure the Datastore Usage alarm.
André