The current default for "Discard old builds" is not to do so. This is the functional equivalent of "please fill the entire disk until everything breaks."
Please consider making the default for new projects 'true'. As for the default number of builds to keep/days to keep builds, I would set one to a small number - probably 1. That way it will be very obvious in short order what is happening and how to increase it - instead of the current situation where everything breaks and it's not real obvious why (what happened? which project is the culprit? etc)
The added advantage is that all that UI/config becomes obvious when the default is true - instead of hidden as it now is. It is far too easy to set up a job and not notice that configuration until things start breaking.