Fibro knitter, checking in! I find that my hands and elbows hurt much more when using larger needles. I'm usually fine with any size smaller than a 5(US). Anything larger and I find myself either having to take breaks or switch to my Denise needles from nickel plated ones. I've also noticed that extended periods of purling really get to me, so I have to be mindful of patterns that call for lots of repeated purl rows.
I also have found that posture matters, but I have found I have more trouble with aching, burning shoulder blades when sitting at my spinning wheel. With knitting, I can recline back onto a pillow, if necessary.