Just came across your question as I was looking for pictures of a fisher. I had one in my backyard a few minutes ago but couldn't get to my camera fast enough.

If you keep your dog on a fairly short leash while walking in the woods you should have no problem. Fishers are very shy and will not approach humans. If you want to let your dog run loose, then you better get a bigger dog. Around here we classify dogs as either diners or dinners, preditors or prey. I would not let a little dog out by itself - even in a fenced yard - with a fisher in the vicinity. But if you live in rural Ontario as I do, then a bigger risk to a little dog (and to you walking a little dog) is the coyote. My neighbour was out walking her five month old golden retriever when she was confronted by a coyote. It didn't attack but it followed her and kept approaching. It would back off if she yelled at it but it wouldn't go away. With both fishers and coyotes the risk of attack is very low during the day but quite high at night.