I use the small end of the dual toothbrush, it's a perfect size for them (I also like the bristles). If they'll let you & they have big teeth (for a Yorkie
) you can use the big end for the surface cleaning on the front teeth & the canines. I also use the C.E.T. small cat toothbrush for their front teeth, they are the smallest toothbrush I've run into. They're a little on the soft side, so I use them for the front teeth.
Amazon.com: C.E.T. Pet Toothbrush: Pet Supplies