Simple question from New guy

Hello, i am just starting to learn Adobe photoshop. I am working with CS2 version. I got a simple question that I can't find the answer to even why I go to the stupid help. Don't laugh, please.

I have two layers of graphic (two photos). One Billboard and one photo of a person. I have managed to extract the photo of the person out of the first picture and now I am trying to add it inside the billboard. I have made two layers and I have one image on top of the other image. Now the problem I am having is scaling the Photo of the person and making it smaller so I can put it inside the billboard. When I change the image size, the whole thing becomes smaller and that is not what I want to do. How do I make just one layer smaller, so it could fit in the object in the second layer?

Thank you in advance...