If you fancy yourself a painter like Picasso or a doodler like da Vinci, then your chance for recognition and discovery is here. There's going to be an attempt to create the world's largest painting on Saturday, and the public is invited to lend a hand -- or at least a stroke or two. The Pennsylvania Avenue Development Corporation is holding a summer festival on Western Plaza (between 13th and 14th streets NW) with gourmet food, live entertainment and a 30,000- square-foot nylon-laminated polyester canvas that will stretch the length of the plaza along Pennsylvania Avenue. The canvas, 600 feet by 50 feet, will require more than 300 gallons of paint and will be divided into six sections lashed together with rope. At 4 the artwork will be hoisted by three cranes and hung vertically for photographers and painters to admire. If you get tired of painting, you can take a seat at the tent-covered sidewalk cafes that will be set up on the plaza. Continuous live jazz, rock, country and Dixieland music will play on the plaza's two stages from 10 until 6:30. Artists should dress to paint and bring their own brush or roller. (If you don't have a paint brush, organizers will lend you one of theirs.) Western Plaza is near the Metro Center and Federal Triangle Metrorail stations. If you're driving there, remember that Pennsylvania Avenue, between 13th and 14th streets, will close at 6 a.m. for the day. Festival hours are 10 until dusk; call 566-1218; free. If it rains, the fun switches to Sunday -- same time, same place.