CUPERTINO, Calif. — As it struggles to match the success of its big-screen iPhones, Apple is now contending that small can be beautiful, too. The giant tech company showed off downsized versions of its signature iPhone and iPad Pro tablet on Monday, hoping they’ll appeal to first-time buyers and those who have shied away from the bigger-screen models Apple has sold in recent years. At a time when overall smartphone sales are slowing, Apple touted its new four-inch iPhone SE as the “most affordable” new phone the company has offered.