Generally, a dog shelter should be at least a foot longer than the dog and it should be as wide as it is long. Also, if you live in a cold climate region, please put a floor in it to get the dog off the cold ground. Your dog will stay healthier if it doesn't have to sleep on the frozen ground.
Now, lay a piece of plywood on the ground cut to the dimensions you want. Make sure it is squared properly and the plywood should be finished on one side, the side facing the inside of the dog house. Next, place the floor joists under the two sides of the floor and nail the plywood to them. Next, place two joists on the opposite sides of the floor so that you now have a joist running all the way around the outer edge of the floor. Then put the middle joists under the floor and nail them. The joists should be three inches shorter than the plywood, an inch and one-half on each end. These two joists should also be nailed to the joists they butte against.
With the floor finished, cut some bottom plates for all four sides of the dog shelter. They will lie on their side
and you will nail the studs to these plates. On a dog shelter that is three feet long, you only need three studs, one on each end and one in the middle. After you get the studs up, place the top plates on them and nail them to the studs. Don't forget to put the entry hole on one wall. You can decide the entry size based on your dog's size.
Once the walls are up it's time to put on a roof. The pitch will be your first consideration. A flat roof is no good. I suggest about a 4-12 pitch. That means the roof has a drop of 4 inches for every 12 inches of length. It allows for good water runoff and is still strong enough to hold a substantial snowfall without buckling under the weight. Cut a piece of plywood the same size as the floor and lay on top of the walls and nail it down. Then cut 2x4's to go around the top just as you did for the floor, except these will lie on their edge. Build the roof just as you did the walls except the 2x4's will be notched where they lie across the top plates. When you're ready to put on shingles, use light colored shingles. These will reflect the sunlight in the summer and keeps the dog shelter from getting unbearably hot. And don't forget to use foam in the attic of the dog house to help keep the heat inside in the winter.
You may even consider putting a doggie door on for the winter. It will help keep your pet warmer by keeping out the wind. You can build a dog shelter for less than $100 if you do the work yourself.