The hailstones are each about one hundred pounds.
We understand that such hailstones falling from a (couple kilometers’) distance are crushingly devastating for any structure man has built in his cities. The size of such hailstones speaks to the ferocity of the storm (“rumblings”) that occurred when the angel poured God’s wrath on the air. The point of course is that God’s wrath is devastating, also for the spirits of the air. Here is a sense of what the Lord means when he says that “Babylon the great” must “drain the cup of the wine of the fury of his wrath” (Revelation 16:19).
21 And great hailstones, about one hundred pounds each, fell from heaven on people; and they cursed God for the plague of the hail, because the plague was so severe.