Dude don't worry about it. If your just starting it really dosn't matter for freeriding or freestyle. The DH is a badass board I ride the 2006 155 model myself. Unless you already know how to handle jumps and rails board setup dosn't comes into play yet because you wont feel the difference in a few cm. As far as a first board for learning your good with the 159 DH just keep riding.
riding to big of a board in the park while trying to learn is awful...its the other way around, learn park on a shorter board and when you get better you can go longer. i like my park board ot be between 150-152 and i weight 165-170