Joyce,
I have two bitches that are 15-19 pounds. They wear a size Zero halti-- it's the smallest size. Supposedly, bitches do have smaller muzzles than dogs, but who knows if it's enough to make a size difference.
If you can't find one small enough or don't want to bother take Kerioak's excellent advice. I"m not sure what type of collar she prefers, but even on small dogs, the chain does work quite well --- if properly used :rolleyes: :)
One of my 'girls' will not EVER get used to her halti, so I use either a body harness on her or a 'choke' chain (proper term -- 'training collar'). On the harness, she will pull. But as soon as I put on her 'chain' she walks a perfect heel.
If I have her regular 'buckle collar' on, she really does forget to walk alongside and starts pulling to go faster :rolleyes: She'll correct if I "remind" her. (meaning just saying her name and she'll look, then ease herself back into position)
..but it gets really annoying after a while :)
Even though I never have to 'snap' her chain, just her wearing it makes her really conscious of staying next to my side. I have no clue why

But it does work :)
Just in case you don't know (if you do, please ignore) : To make sure the chain collar is going on the correct way,
when you place it over the dogs head, (and you are looking towards the dog's nose) it should look like a letter "P". If it looks like a "9" it is upside down. When you pull on the "dangle" part and release it, the collar loosens up automatically.
hth and good luck
toodles