What should I put as my available disk space?

[Alexey]

as written above:

@Alexey

the safe would be to use results of df -h (it will give you TiB value), but specify it as TB in the storagenode config.

This will leave approximately 10% free space on your disk.
But I wouldn’t specify more than 14.4TB at the start. Your node would need a time to fill even that amount.