Dormers are one of the hardest things to get right on a building. Too small and they look like Zippy the Pinhead (link here?) and too large and they look like Andy Rooney's eyebrows. Proportion is everything when it comes to dormers.
One of the This Old House TV show houses had dormers with bad proportions, and Tom Silva came up with a reasonably inexpensive fix for the problem. You can read about it here
It's not unusual for your customers to want a dormer so they can convert and attic or the space over the garage into living space, but they might not know what style of dormer they want. Ask them to drive around with a digital camera and take shots of what they like.
And then send them a link to this gallery and have them take a look at the different types we've collected here.