More than 5,000 sailors and marines aboard the nuclear-powered aircraft carrier Harry S. Truman arrive in Key West today for four days of shore leave. The HST is the United States' largest aircraft carrier. The huge vessel will anchor eight miles out and the crew will be ferried in to shore. The first contingent should be arriving about Noon, a spokesman said. Three-to-four thousand sailers may be on the idand at any one time.