I would recommend you to stop thinking how much usable space on a disk, but instead ask how much overall usable space you'll get after AFF/FAS system configuration.
The reason I'm telling this because there are many factors and configuration choices in your particular case which can influence overall usable space regardless of usable space on a single disk.
In many cases it is easier to go even vice-versa: tell how much usable space do you need and what type of data you are going to store, and from that, we can find what storage system suits for you.
The first place to go is to ask your NetApp representative.
If you have a partner account, you can go yo synergy which can help to find usable space for your storage system configuration.
cluster1::> storage disk show -raid-info-for-aggregate
Owner Node: node1
Aggregate: aggr0_node1_0
Plex: plex0
RAID Group: rg0
Usable Physical
Position Disk HA Shelf Bay Chan Pool Type RPM Size Size
-------- --------------------- ------------ ---- ------ ----- ------ -------- --------
data node1:2d.11.2 2d 11 2 B Pool0 SAS 15000 9.77GB 9.93GB
dparity node1:2d.11.0 2d 11 0 B Pool0 SAS 15000 9.77GB 9.93GB
parity node1:2d.11.1 2d 11 1 B Pool0 SAS 15000 9.77GB 9.93GB