Dog shark sweater question

I recently purchased “the Cool Girl’s Guide to Knitting” because it had a pattern for a dog sweater that made the dog look like a shark. I have looked over the pattern but they do not give the finished measurements merely small medium and large. The dog pictured is a jack russell which is the dog I will be making it for, but I have no idea which size to make. The default pattern is for a small dog so do you think it is safe to assume that the dog pictured is the small size? Has anyone out there used this pattern before? Or is there a way to figure out how big the finished measurements will be?

I don’t have the book, but you probably can get an idea of the measurements by dividing the number of stitches by gauge. If you can figure out how many stitches it is around the body, for example, from the pattern, and if it’s 5 stitches per inch, then you can divide the number of stitches by 5. Did I make myself clear?