: I tend to imagine the Nibin to be slightly smaller, with hair all over, and more muscular (like the traditional Scottish brunaigh or brownie); but I don't know if I read this or if I made it up in my imagination and just thought I read it.

Yes, I think I remember the Silmarillion saying they were 'of lesser stature' or some such, along with some other detail. That's where I would look for an answer.