Animals missing from Natural Order Scale

I’m probably not seeing some section of the book. How do I determine the category that an animal is in the Natural Order Scale? A lot of animals aren’t in the list or chart.

Look for an animal of a similar size that is on the chart.

That’s what I kind of figured. Looking through most of the animals not on the list that I’m considering seem smaller than a beaver or an owl, so it’s kind of a moot point, since then they’re small enough to be killed by a mouse.