Help resizing Photoshop project!?

Help resizing Photoshop project!?

I am creating a business banner. I used the wrong dimensions (although it was fairly large) instead of the 4' x 8' that I was supposed to use. I have my original PS file with all my layers. How can I bring those layers over to my new 4' x 8' project so I can work on sizing and aligning everything?

Thanks so much!

I'm using PS CS5 extended.
The amount of times I've re-done a piece and there was a solution as simple as this. Never even attempted to check whether it was possible, duh. This will surely come in use, cheers.