In the upper left hand corner of Finder, in the toolbar, you'll see an icon that looks like
Click it.
Move your cursor to in between the columns, and click, hold it, and drag to whatever length you want.
This is something that former Windows users ask quite often here. The answer, OSX when you click the green button does not expand the window accross the ENTIRE SCREEN and waste space, instead it smartly resizes the window to effectively display whatever it is in the most space-saving way. It takes windows users a long time to realize that it's actually BETTER this way. Of course on some applications (Garageband) it ends up that a full-screen window actually IS better than a smaller one, and OSX recognizes that.
However, if you MUST have you're window the entire screen. there will be a small diagonally striped triangle at the bottom right hand corner of all windows. You can click and drag to what ever you want. Even full screen.