The Puranic declaration is that the Lord himself causes these chakras in the guise of worms. He reveals his presence in Saligram through the Chakras. Because he is already present there by his own choice, we do not perform any pratishta-ritual for Saligram just as we do for other idols. According to 18 Purans, a good number of them, under different contexts, proclaim the Mahima of Saligram as Vishnu pooja. The size recommended for the puja in the house is the smaller ones, which can be accommodated, in the closed fit. Big size saligrams are to be kept in the Temples/Mutts etc.
Specification of Shila:
Weight: 73 gms
Dimention: 25.73mm (L) X 59.25mm (B) X 29.79mm (H)