As I read it you are running into the aspect ratio and don't quite get it.
First of all open the Image/Image size and see what the document size is. You can resize this bigger and smaller but the aspect ratio stays the same.
So if you have a 4x6 inch document and want a 3x6 you have to crop off the width to make it fit. If you do not want to crop anything you can distort the picture by stretching or compresssing one dimenstion, but you will have a distorted picture.
The other option is to add to the height by adding more to the picture. I believe this is what Scott was trying to do with Content Aware. Making new pixels to add to picutre. If it is simple like a sky it can work easily, but more complicated pictures it may be difficult to acomplish.
Just remember to keep the same aspect ratio you have to multiply the height and width by the same number. If you print size is something different you either have to crop the image or create part of the picture where none existed.
When taking pictures and you are not sure how you want it to print take several pictures of differenct zoom magnification. You can always crop the way you want it if the resolution is good.
If the original image size is adequately large in pixels to print the quality you desire at the largest size already,
then you probably don't need to resize at all.....just use printer settings to print at a smaller size....assuming the aspect ratio (length/width) remains constant.
If the aspect ratio needs to change, then cropping may be needed.
Regardless of anything you do....always keep an original saved as backup in case things go wrong and you need to recover.