A week or so ago there was a discussion about the behavior of the Finder and how it handles default window positions.
John Gruber of Daring Fireball linked to a post by Christopher Phin (www.chrisphin.com) explain how to set the default for Finder windows.
Here is the post.
Here is relevant bit of info:
To have Finder windows in OS X open at a consistent size and location, open a new Finder window, resize it to how you want, then, and this is the important bit, close it again before you do anything else. Just open the window, resize and reposition it, then close it. Don’t click icons inside it. Now, subsequent new windows will be at the same size and position.
Nice and simple.
Take a look at the post as he has a few other tips.