Well for the price, it will take a little sewing to shorten the part that goes behind the head for the small dog, in the tight plus, it was too long for my Dwarb pinscher.
I think I took size 2 and size 3 to be sure, one is just in terms of the length of the snout, it touches the end and size 3 is not bad, even though I had to shorten the strap of 2.