If the chicken breast halves in your market are larger than 4 oz., they will take longer to cook. Be sure to cook them long enough so that they are no longer pink in the centers and the juices run clear.
How to Safely Handle Raw Chicken
Always wash hands, preparation surfaces and utensils in hot, soapy water before and after contact with raw chicken. Dry hands and surfaces with a clean towel or paper towels. Take special care in washing worn cutting boards as there may be hard-to-clean grooves or cracks that can collect bacteria.