I am always on the look out for smaller sized backpacks, my son is 24 hour tube feed, so we need items like this to carry his pump and milk- and that are small enough for him to be mobile while wearing.
My son is 3 years old, he is 33 pounds and 34 inches tall- maybe that helps with trying to figure out the sizing. The middle photo is him wearing the back pack.
It is very roomy- the front, where the doggy mouth is, is a zipper compartment. There is a pocket on the inside and the rest is just open area.