fix(rxDiskSize): Properly handle 0 size with no units specificed

Closes #636 

If we couldn't find the unit in our lists of units (which would happen when no explicit unit was given), then we called `Math.log(size)`. If `size` also happened to be 0, then we'd get an error, because you can't do a log on 0.
Replace 'TB' with 'PB'.