in this wiki [Xiao Ning Er Appearance and Personality part] it's described she having a [Short, Shoulder-length, Black] Hair [wiki]
but in Novel ver....
[c4] [Her black hair draped over her shoulders]
[c6] [her shoulder length hair]
[c9] [Her ink black hair scattered around on her back]
[C18] [Pure black hair cascading down, studded with silver ribbons]
none of it say that her hair is short, and if her hair could "draped over her shoulders" & "scattered around on her back" her hair is clearly not [Short, Shoulder-length] like [Irisu Makina - Grisaia no Kajitsu]
her [Shoulder-length] hair probably pretty long, might be similar with [Yurippe - Angel Beats!]