What Lenght Needle?

What length of a size 9 needle do I need to knit a sweater back which is 83 stitches using medium weight wool?

Whichever length the sts will fit - it could be 10", 12" or 14", depending on how squished together you don’t mind the sts being. Or you can get a circular needle and you don’t need to worry. You can knit flat on them and they hold many more sts than a straight needle, which is good if you want to knit a blanket or shawl, as well as a sweater in the round. A 29-32" long one is a very good universal size.

I tend to use my circs for everything but like Sue said, you use whatever length feels better to you. I hate using the long 14" needles so for me if I am using straights if it doesn’t fit on at least ther 12" I would definitely go to my circs.

Thank you both so much for the help!

I would also probably use circular needles. I do like straight needles, but I find that that many stitches gets heavy after a while. Having the stitches sit in my lap (as they do with a circular needle) is easier.