Recommendations for "Faschen" sought - nostalgic charm

Does a border also count as a fascia? I think you can be bolder than 6cm with the width. I assume the "inventor" was aiming for the optical enlargement of the windows.

How wide are your reveals?

That cannot be answered in general / transferred to your project. Show your facades. I would also base it somewhat on the color contrast, and a bit on corresponding sizes such as e.g. roof overhangs.