How immense must the destruction of Hiroshima been for the Enola Gay s crew to have been able to see it?

Well, they were over 8 miles away when the bomb detonated, so they wouldn't have seen any of the destruction. Even reconnaissance planes were initially unable to survey the extent of the destruction due to the enormous amounts of smoke caused by the fires, and dust brought up by the bomb.

Also the service ceiling for the B-29 is 31,850ft. 40,000ft would've been impossible.