Are you talking about the physical size of the image or the size of what displays?
I adjust image size using Paint Shop Pro, although there are a number of free image adjustment tools. Display size only needs height and width attributes. You can also resize the image.
Before you ask what the difference is: let's say you take a picture of your pet dog. In the editing tool, you can select the dogs head and use that as your image, or you can adjust the height and width of the image.
Display size displays whatever the image is either larger or smaller, depending on the height and width values.