FREE Coffee Mondays at McDonald's...   OAK BROOK, Ill.---- McDonald's Corp. said Thursday it will offer customers a free iced or hot mocha every Monday from 7 a.m. to 7 p.m. from July 13 to Aug. 3.   The complementary beverages -- a 7-ounce iced mocha or an 8-ounce hot mocha -- are smaller than the "small" 12-ounce size available for purchase. The free drink sizes are only available on Mondays.   "This is one of the largest sampling initiatives we've taken on as a company," said spokesman Neil Golden in a statement.   McDonald's has been touting its new espresso-based drinks since last summer when the chain began introducing them to menus in all the company's U.S. locations. The nation's No. 1 hamburger chain has said the drinks are already adding to its revenue.