Medium sized dogs are a great choice If a big dog is just too big and the little dogs are just too small for you, then you may want to consider getting one. English shepherd- http://encyclopediaofdogbreeds.com/medium-dog-breeds/
Medium coat, red merle Koolie is an Australian dog breed. The Koolie is a working or herding dog which has existed in Australia since the early 19th century when it was bred from imported British working dogs.
Australian Shepherd: These beauties aren't actually Australian (even though there are now breeders in Aus) and are renowned for their intelligence and good nature. They're a medium sized pooch recommended for active families with lots of space. Perfect as a working dog but also more than happy in a family environment provided they get enough exercise and mental stimulation.