If my testings are right, then the Blender 2.79 background image size refers to the image extension along the X-axis in Blender Units. The default size for background images in Blender 2.79 is 10 Blender Units , whereas the empties in Blender 2.8 with display set to “Image” set the default size to 1 Blender Unit. The image position in both cases is the location of the image center. I you add an Image Empty with ADD => IMAGE => BACKGROUND/REFERENCE the default size is 5 Blender Units. As @spikeyxxx mentioned it above, its half the default size of Blender 2.79 Viewport Properties Panel’s background images.
No matter whether you add in Image Empty with ADD => EMPTY or ADD => IMAGE (BACKGROUND or REFERENCE), you can later convert one into the other by changing the settings in the empty’s “Object Data” tab.
I also had issues with Blender 2.79 background images in terms of blurryness and of image shifting relative to curve objects (used for contour map tracing) in front of the image when zooming in or out (See my post here).
