Tiled tunnel under Belgian river "staggeringly monotonous"

After the delightful wooden elevator down from Antwerp's streets, the 1876-foot pedestrian tunnel under the Scheldt is "staggeringly monotonous," reports Atlas Obscura, but I rather like it. I have one question: what would the diameter of a well-lit circular tunnel like this need to be for its curvature to be imperceptible?

Actually, I have another question: What would the diameter of a dimly-lit circular tunnel like this, lined with books, need to be for its curvature to be imperceptible?

Photo: Trougnouf (CC BY-SA 4.0)