Stout mentioned the cultivar 'Gracilis' as having narrow leaves in his book. There are a couple of mentions of species having narrow or wide leaves but in most of these it was in relation to height, and that's a ratio that would be a factor - you wouldn't expect a small, short daylily to have wide leaves compared to a taller one necessarily. There was one where he mentioned leaves being almost an inch across which he considered "coarse" for the height of the plant.
If we go with "coarse" equating to wide, he considered Hemerocallis citrina to have "coarse" foliage and the fulvous daylilies to have "medium coarse" foliage. Hemerocallis dumortierii is the one that he considered to have "coarse" foliage for the height of the plant, leaves an inch wide. The cultivar 'Mikado' he considered "medium coarse". 'Nocerensis' is described as having coarse foliage but scapes 4 feet tall. 'Margaret Perry' he considered to have coarse foliage and 'Soudan' medium coarse.
If anyone else wants to search the book, the pinched nerve in my shoulder is starting to protest so I'm stopping for now, it's online here:
http://babel.hathitrust.org/cg...
