We have two dogs, both about 4.5 pounds and we use a infants toothbrush. The best one we found is Oral B because the head is more round than long and oval. Also, you could use a smaller one by just buying the head of an electronic toothbrush. Bandit already has buildup but isn't horrible with the toothbrush as long as its less than 30 seconds, but the puppy hates it so we're trying to get her used to it now.
Good luck
