What does it mean when an anime character has small eyes?
According to TV Tropes the physical trait of "eyes always shut" is commonly used for the "wise guy" character type, in which case they'd be referred to as "kitsune No me," or in English, "fox eyes"/ "shifty eyes."
Depending on the way they are drawn the eyes can represent anything from serenity, dignity, smugness, humor, indifference or even someone trying to fake any of those.