Make the frame as small as practical. So many people ride needlessly heavy bikes because they get sizes larger than they need.
Wrong. Most people buy frames too SMALL. Saving a few ounces and ending up with a poor fit is simply ******ed.
If you want a light bike I would get a decent light steel frame, a carbon ebay fork, and a lightweight drivetrain. Most of your weight is going to be in the crank/BB and rims, so spend your $ there. I built a Konda Paddy Wagon for a friend, and that bike was light, and it doesn't have the downsides of aluminum or carbon fiber.