Hi Anna, I use the Garmin system for my pointer dogs and it's a great system. I don't know if you can adjust the collar size for terriers but it is expensive to buy and probably much of the information (dog speed, km travelled) is not necessary for you. The accuracy is great and you get good signal in the forest so it is possible to almost always know where the dogs are.
I first saw this system used in Norway for elk (moose) and it is obvious when the dog is working a hunting pattern or if the dog is chasing (travelling only in a straight line). It also tells you if the dog is stopped (bay