When your quilt is done, measure its width and length. Once you have those numbers, you can go buy batting; check the package to see the actual measurements of the batting and pick some that is slightly larger than your quilt. Different manufacturers make queen batting in slightly different sizes, so if your quilt is a little larger than the batting one manufacturer makes check another, you may find the perfect fit. King size will of course be plenty, but why buy king sized batting if queen sized would be big enough?
I like the whole line of Quilter's Dream batting and also American Spirit Luxury Blend, 50/50 bamboo/cotton, which is a new brand from Fairfield.