While Levain Bakery’s rich, buttery cookies are humongous — each weighs a whopping half-pound — its 400-square-foot Upper West Side home is uncomfortably tiny. After 21 years, it will move two blocks away, to a larger 3,000-square-foot space at 351 Amsterdam Avenue, but it will still keep the 74th Street original. This will surely mean more cookies for all — as well as, most likely, expanded operations. (New cookie flavors? Additional kinds of other pastries? Wholesale potential?) Take note that when Grub recently ordered a box for local delivery, the cookies arrived still warm.