Minimum array disk size

Was wonder if anyone has a array with a 16GB drive and how much space is usable on it. I know that the used space on a empty drive goes up the bigger the drive and I can see on a server I have that a 2TB drive with nothing on it is using 14GB so was wondering how much space would be usable if using a 16GB drive. Asking because I have a spare 16GB Optane SSD & wanted to replace a HDD I'm using since I don't need much space.