I've never actually done this before, and I know that there's more to it than just slapping some pieces of wood together. I live in a place that gets pretty much all the extremes of weather, including extreme dry heat, extreme cold and snow, very little humidity all year round, and yearly extreme wind. Though rain is scarce, when it does rain it does with a vengence! :) But besides weather and the dog's size, what else is important to consider when designing it?
I usually build people houses, but have built a dog house or two. My research says that the size should be adjusted to the size of dog. It wants to be just big enough for the dog to lay down stretched out, and not too tall. A raised insulated floor is good. The reason for this is that the only heat source is the dog's body heat and too big a house will be too cold in winter. Of course, it should be insulated and have a safe sheathing on the interior to cover the insulation. It should also have some type of flap door that the dog can easily pass through, but will reasonably seal the opening from the elements. I have seen several books with designs and decors for your pet's condo at building materials stores and book stores online. Don't forget to put it's name on a plaque over the door!
PS: If the weather is really bad, let the poor thing indoors, have a heart.