If you get a scanner, most scanners let you scan at a smaller size.
Your photos are really good though for detail. If you want to figure out about squaring them better, keep in mind that the larger side of the resulting image is what the camera is closest to. In other words you are not holding your camera parallel to what you are photographing.
You should be able to see it through the view finder. If the top of the old photo looks smaller than the bottom, it means that the camera is closer to that side. You need to get above the center of the photograph to level it out. A tripod can help you with this. You can see that the lens is parallel by stepping back from your set up before you take the overhead photo. And you should see the same in the viewfinder or screen. Same goes for one side bigger than the other. Your camera is more to one side than centered.
To help explain what is happening: When I take a photo of one of my paintings, I lean the painting on the wall of my porch out of the sun, so I get the light reflected from the sky. When I see in my viewfinder that the top is bigger than the bottom, I know I have to lean the picture even more so the top recedes. That or I have to move the camera lower so it's 'closer' to the middle of the painting and not the top.
Anyway nice photo reproductions!