Shannon,
My rottweiler Crash is the same way with little dogs. I think he thinks that they are cats (and he does NOT like cats). His main target are pugs. DH and I take Crash and Tyson to the dog park every weekend. What I've been doing, is if he gets near a pug I make him sit and give him a treat. If he starts sniffing the pug and doesn't try and knock it over, I give him a treat. As long as he doesn't try and plow over the poor thing, I give him a treat so he'll start to associate small dogs with getting treats, which would make it a positive association. I do see some progress, although at times he still gets a little too rough with them and I just call him to me and move to a different section of the park.