It's tricky to design gable dormers so they look just right on the house. Here is a piece I wrote for Fine Homebuilding magazine on how to design gable dormers, also known as dog-house dormers. As an example, we used the editor's own home, which is fairly traditional and called for traditional dormers. Two dormers looked too sparse, large dormers are very rarely a good idea, and a nice low-profile shed dormer didn't interest him. By keeping the dormers small and balanced on the roof, they bring in light without overwhelming the house.
Read the article here.