I think you need to make a conscious effort to really reward her for being calm when she sees smaller dogs when you're out and about town. Don't get too close necessarily, but when you see a small dog, give her something yummy she doesn't normally get. She will begin to look forward to seeing little dogs.
Although it's fun to go places with your dog, it's a lot less stressful on them to just stay home if they are constantly going to be harassed. And less stressful for you.