I am preparing a 3.5 pound beef brisket, and using a recipe that anticipates a 6-6.5 pound brisket. How will this affec the cooking time?

Brisket is a cut which demands slow cooking. But a lot of it depends on what you are doing with it; it could be anything from corned beef to pastrami. The cooking time should be close half for the six pounder but more information is needed.