Business cards? This is an extremely generic question, and would help if you could tell us what the question is. However, looking through our list of sites, I only think that maybe Graphic Design might work, but it's a stretch.

What is the ideal width for very high-quality professional business cards?

This is not on topic anywhere on Stack Exchange. You'll need to do research yourself on this one. It doesn't really fit any of our sites and is very broad. It also differs per each person.