Why were the windows on the Concorde about the size of a hand?
The Concorde flew above 15,000m. At this altitude a sudden reduction in cabin pressure would prove hazardous to crew and passengers with most falling unconscious within a few seconds. The low air-pressure would also render the oxygen supply system inefficient and most passengers would suffer from hypoxia.

Thus we have the reason for the smaller windows, should a breach occur then the size of the hole being small would reduce the rate if loss of air-pressure inside the cabin. This, combined with a reserve air-supply to augment the cabin-pressure as well as a rapid rate of descent manoeuvre to bring the aircraft to a safe altitude would reduce the the risk of hypoxia.
So basically the small windows were designed to reduce the rate of air escape from the cabin should a hull breach occur.