Government docs from 60s stated that 10 sq. ft was sufficient. One picture shown was someone sitting on the floor with knees pulled in to the chest, and under a table that had been reinforced-improvised shelter.
For a planned renovation, there is a 10x10 concrete block area the basement, and ceiling reinforced with a diagonal (cuts the space in half) with the maximum protection in the corner of 50 sq. ft. and that was for 5 people.
Such shelters sound like they are intended for blast protection, not for a living shelter while radiation drops off. While these would probably let one survive the blast near ground zero, if you are down wind of the blast, I doubt you could scamper to safety.