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A sample ordinance from the Humane Society of the United States indicates a run should be 3 feet wide by 10 feet long and 6 feet high with the width increased by a foot for dogs over 100 lbs. Multiplying a dog's length by five can also be used to figure a run's size.
According to the Animal Welfare Act Regulations, enclosures for dogs and cats must, at a minimum, "provide sufficient space to allow each dog and cat to turn about freely, to stand, sit, and lie in a comfortable, normal position, and to walk in a normal manner." While this doesn't give dimensions, it does indicate a fairly small space.
Give your dog the largest run that your yard and finances will support, as long as it is above the Animal Welfare Act Regulations minimum. Also factor in how many dogs will be in the run and how often, whether or not a dog house will take up some of the room and any safety concerns you may have.