Finder windows on OS X are a bit finicky. One time you'll open it and it will be tiny, another time it will be huge. Over on MacWorld, they take a look at why this happens and came up with a way to set a more default behaviour.
The Finder window behaviour can get a bit frustrating if you like your windows to open up at the same size every time. So, MacWorld tried to come up with a way to get consistent behaviour. It's a bit weird, but it seems to work:
- Open a Finder window and set it the size you want
- Close all windows (Command+Options+W and click the close button)
- Hold down the Option key, right-click the Finder icon, and select relaunch
Now, you should be good to go from here on out with a persistent Finder window size.