How big does an asteroid ... yes I know there are rocky (solid) ones and not so solid ones ... but how large does an asteroid have to be in order not to burn up in the atmosphere and therefore cause some significant damage ?
It probably has to do with how much enthropy it can absorb.
Probably on the order of meters, depending on material.
It's not the size that determines that, so much as the angle of entry.
The asteroid that is believed to cause the Tunguska event is estimated to be "a few tens of metres across" - and 23 feet is 7 meters. The energy of that explosion was estimated to be 10–15 megatons. So you can sleep well, a mere 23 feet asteroid would release only about a megaton, nothing to worry about :-)
The Tunguska Event was a powerful explosion that occurred near the Podkamennaya Tunguska River in Russia on June 30, 1908. Although the cause of the explosion is the subject of debate, it is commonly believed to have been caused by the air burst of a large meteoroid or comet fragment at an altitude of 3 –6 miles above the Earth's surface.Different studies have yielded varying estimates of the object's size, with general agreement that it was a few tens of metres across.
Estimates of the energy of the blast range from 5 megatons of TNT to as high as 30 megatons of TNT of TNT — roughly equal to the United States' Castle Bravo thermonuclear bomb tested in late February 1954, about 1,000 times as powerful as the atomic bomb dropped on Hiroshima.
So, "a few tens of meters" - say 30 meters or roughly 100 feet across. And the rock that passed by today was about 23 feet across - maybe 1/64 the mass of the big Tunguska rock.
The photos of the flattened trees from the airburst are really incredible.
I don't think a 23 foot diameter rock would burn up in the atmosphere.