Offering the service as a custom road bike builder is quite a privelage. Someone is entrusting you to make the bike of your dreams and not just getting a bike in a box.
When we build a custom bike we spend a lot of time talking it over with the client and finding out how they ride, where they ride, their dimensions and their preferences, plus what we believe would be a good match.
Obviously frame choice is crucial in a road bike and the geometry varies from brand to brand, but now consider how that geometry relates to your leg length, torso and arm reach.