the one he is using is like what 25-30 cm? on this french website they have an atlas stone of 20 kg (40 pounds) (that's their minimal weight, they go up to 110 kg = 242 pounds
), for a circumference of 81 cm, thus a diameter of 81 / 3,141= 24 cm = 9,44 inches. But it is marble, 220 euros
.
Powergym vous présente en exclusivité sa pierre de l'atlas en marbre, pour les amateurs de strongman et d'haltérophilie. La Stone ball de l'atlas est proposée en 6 poids différents : 20, 30, 40, 60, 80 et 110 kg.
www.powergym.fr
Else there are some decoration sphere for garden, there is a website that sell 20 cm (7,87inches) granit diameter sphere, for 79 euros, or marble one for 99 euros. I've contacted them to know the weight. They also sell 30 cm diameter.
regarding the goal one wants to achieve, there can be only one Marin, having big arms to impress the girls on the beach...