We have a fence from Innotek (bought from Cabelas).

Let me just give you a visual of how strong the shock is.... (Note: at this time it was only set on 1/2 power)

When I was testing where the parimeter of the fence was, I was holding the collar with two hands and forgot to change the prongs to the rubber ones. Somehow I had made contact with each prong when I crossed the parimeter. It felt like two 300lb guys standing on each side of me and punching me in the shoulders at the same time. It hit me hard, really hard.

Now, Let me just tell you one thing. At the beginning, the dog got through the barrier once, to no fault of his own. It was poor placement of the wire on my part.

If you get one, you'll see how well the shock correction works for your dog (They probably won't get corrected (shocked) more than 5 times before they learn). On full power it should work for any size dog as long as the prongs are making contact with the dogs neck.