What are you sheltering from? In most situations small is beautifull. If you are cold the smaller structure will heat up and retain warmth far better than it's larger equivelent. For longer term shelters the exmples used by the native peoples are hard to beat. Taking into account location of the shelter and natural features to incorporate will also help. Building a shelter in a low spot out of the wind just to freeze in a cold sink or facing opposite the rising and setting sun are a few examples.