TELEVISION - Feb. 21, 1992

Having Their Say: Larry King will question Keith McKennon, chairman and chief executive officer of Dow Corning Corp., about breast implants on his CNN program tonight at 6. . . . On ABC’s “Nightline” (11:30 tonight), Ted Koppel will talk with cosmonauts as they orbit the planet for a country that no longer exists. . . . “Entertainment Tonight” (Channel 4, 7 p.m.) screens the public-service announcement Paul Reubens (Pee-wee Herman) made as part of his no-contest plea on lewd conduct charges.