Eyes that are narrow. Either because they have been closed half-way, doing Jitome, or are naturally narrow, without any physical exertion.
Not all half-closed eyes are narrow, as some images show very large eyes that would merely be averagely sized when half the lid covers them (by anime standards). Same goes for most jitome.
Examples
- Left: narrow eyes, Right, average eyes
- narrow due to being half-closed