All quilt backing and batting should be squared up and pressed, and need to be at least 6" wider and 6" longer than the quilt top.
You need to consider what your quilt will look like after it's washed. All batting made of cotton or cotton/polyester blends will shrink when washed, some more than others. If you have pre-washed your quilt fabrics but don't pre-shrink the batting, your quilt will most likely have that wrinkly, "old-fashioned" look after washing. It will also shrink up to perhaps 8%, depending upon the brand of batting, so keep that in mind as you plan the dimensions of your quilt. Polyester batting will not shrink significantly.
If you decide to pre-shrink your batting, follow the directions on the package. I do not preshrink batting.