SAN volume - Datastore - and VMDK considerations

I created a 2047GB volume on my SAN. I created a datastore in vCenter utilizing all space. I now need to add a disk to a VM using that datastore. What is the best practice in sizing VMDK on a datastore? Dont you need overheard, if so, what precentage of the datastore should be the maximum disk size?

Thank you,

TJCooper