(Will these hailstones be about the size of a beachball?)

Revelation 16:21 And there fell upon men a great hail out of heaven, every stone about the weight of a talent: and men blasphemed God because of the plague of the hail; for the plague thereof was exceeding great. 

(If so, then they will probably kill any creature they directly hit.

Also, hailstones that big, would probably smash through roofs of cars and buildings.

Terrified people would definitely seek to run for cover, but to where?

Would they possibly be safe in a cave, or in lower basements, or in a subway tunnel?

However, very shortly before the hail, there will have been an horrific earthquake:)

Revelation 16:18 And there were voices, and thunders, and lightnings; and there was a great earthquake, such as was not since men were upon the earth, so mighty an earthquake, and so great.

(So, the places that might have been a safe place to flee from the hail, may already be destroyed.

If there had been people in those places, they may have been crushed in the earthquake.

Or there may be countless numbers of people buried alive, and possibly injured, as well.

Emergency services, may be called out to try to find and rescue these people.

But, as they are busy attempting to rescue people, suddenly the killer hail comes down on them.)

(People affected by this horrible hailstorm, apparently will not call out to Jesus for help.

Rather, they know he is doing this, and they blaspheme Jesus.

And if the earthquake described with the sixth seal, is another view of this same earthquake,

then they admit that it is Jesus, and they are terrified of him.

But, they don't repent, like the one thief on the cross, who did.

Rather, they die, angry and impenitent, like the other thief, who did not repent:)

Revelation 6:12a And I beheld when he had opened the sixth seal, and, lo, there was a great earthquake;

  14b  and every mountain and island were moved out of their places.

  15 And the kings of the earth, and the great men, and the rich men, and the chief captains, and the mighty men, and every bondman, and every free man, hid themselves in the dens and in the rocks of the mountains;

  16 And said to the mountains and rocks, Fall on us, and hide us from the face of him that sitteth on the throne, and from the wrath of the Lamb:

  17 For the great day of his wrath is come; and who shall be able to stand?