Well, I know this isn't GarageBand but with Safari to 'preset' where new windows open all you have to do is resize a newly opened window, and then the next window you open will be in that size. I would assume it works for GarageBand too.
I am having the same problem--I use a MacBook, and when you open GarageBand, the window is larger than the screen, and cannot be resized smaller because the control in the lower right of the window that allows you to resize the window is off of the screen. When you press the "zoom" control, or the green plus arrow in the GarageBand window, it zooms to the maximum width, which leave the height of the screen still off the bottom of the screen. It basically means that I can't use the program for anything I would like, because many of the necessary controls are at the bottom of the window. Any help would be much appreciated.
To be more precise: I can see my complete GB window even if I go down to a resolution of 1024 x 768, which is way below what a MacBook can do. Have you tried pressing the green plus button in the upper left window corner to resize your window?