Different perspectives and different lenses on cameras, can give no real clue to how large or small a pet might be. My own style is to focus on chest up photos. Unless you are familiar with various breeds, you will not get an idea of the size of an animal from a head shot.
What to do? Simple. Add a person.
In an outdoor environment, pulling back from the photo and taking in a larger area, again with a person, gives a "normal vision" perspective on the size of things. I used a 70-200mm lens for the shot below, which actually was much larger. I cropped it down as to not overwhelm the subjects. You are able to easily see the size of the dog, relative to his surroundings.
It seemed perfect for this blog theme, because our Billie is not only set against the feet of my husband, but she is measurable against the green railing and buildings of Dalton, a familiar sight to anyone from my area. Can you guess how many pounds Billie weighs from this photo?