It’s not a dumb question at all! You would need to use a circular needle. Since the finished dimensions are 30" square, I would look for a circular needle probably a little bit longer than that to make sure it can stretch that width. Since you need size 9 needles, you’d look for a size 9 needle in say a 32" size (maybe even a little longer).
Here’s some help I found with knitting with circular needles:
How to knit with a circular needle: flat (not in the round)
Q. Hello, I am trying to knit an afghan using circular needles. I am quite used to using straight knitting needles but thought since my project is a large one, I would try using circular needles. I cannot seem to figure out what to do once I am finished with one row as I do not want to connect the sides together.
A. Pretend the two ends of your circular needle are not connected at all, like you snipped the cord in the middle. It’s exactly like knitting with two straight needles, the needles swap hands at the end of the row.
If that still doesn’t make sense then read on, I’ll break it down:
After the stitches are cast on hold the full needle (with all the cast-on sts on it) in your left hand and put the empty needle in your right hand. Knit one row. Now you’re going to change hands, as if you were knitting with two straight needles. Put the full needle in your left hand and the empty needle in your right hand, and knit (or purl) another row.
At the end of every row just have the needles change hands so the empty needle is in your right hand.
If you put your knitting down at the end of a row and come back to it later remember to put the full needle in your left hand and the empty needle in your right.