The Preview Image Size differs from the general image size. Thus, it is very well possible that you can find both information in the exif information of your photo, picture or image.
Some images, as well as videos, may contain information about a thumbnail or preview image for the file. This preview is, sometimes, saved as an extra or additional little image inside your bigger image file. In other instances, the image is just a scaled-down version of the original image. In this case, it's not saved in addition to the original image but simply displayed with the smaller resolution.
The Preview Image Size gives the resolution of this thumbnail or preview image, no matter how it is actually stored in your file. The format is, usually, width x height and given in pixels (e.g. 300x150).