I think the capacity of the thing is 3L however, if the fluid goes down the pipes to get to the back window, make sense that you can fit more. I always fill up the tank so I never though about the sensor. But I'm not surprise it doesn't have it.. They didn't think about backlighting the infotainment climate and volume, let alone a washer fluid warning.
It will be 1 year in mid August and I'm already thinking of selling it.