18-20 lbs, like other terriers thinks he is a big dog and have seen him take on a large coyote. Like Jack Russell Terriers, they can run all day. I have taken mine on many trail runs ~10 miles and he wants to do more. They are supposedly by far the best terrier breed for kids and he has lived up to that. Babies and toddlers can literally pull on his face, whiskers, and drool on his eyeball and he honestly seems to enjoy it. He goes right up to little kids unlike any of the breeds I have had before. He is the first smaller dog I have ever had and I have to say I prefer the smaller stool samples in the back yard. But like the other breeds you have listed, he is high energy. He wants to run daily and as soon as he gets home, race out in the backyard to patrol, then immediately comes back in the house with a ball in his mouth.

For a small dog, he is smart, but nothing like my previous dog -- an Australian Shepherd -- that concentrated on people conversations, picking out the words he knew and reacting accordingly all while his eyebrows went up and down. That dog must have had a vocabulary of 400-500 words. By far the smartest dog I've owned and probably the sweetest and most athletic. I've also owned a Saint Bernard (vicious breed), Afghan (incredibly smart and sweet), Irish Setter (faithful and loving and a natural pointer), German Shepherd (temperamental), black lab (kind of worthless but a good pal), malamute (chicken thief got shot by the neighbor), springer spaniel (stupid and worthless) and various mutts.