I had to get the larger "stubborn dog" collar for my boxer. They also suggested shaving the hair in the area of the prongs in the beginning if you are not getting results. The boxer would endure the shock occasionally to get out to run. Our cocker is more sensative (and not very bright) and he refused to touch the grass in the front yard for fear of the shock. We now have 2 acres fenced for the dogs and they will still test the fence line so we ran it around the chain link and it has been a blessing! The stay off the fence line and no one is trying to climb or jump out. If you decide to get one - check Craigslist - I have seen them used on there.
Good Luck!
Good Luck!