The batting should be larger than the top to account for any shrinking of the top due to quilting.

I piece my backing larger than my batting so that I can see it while quilting. I've had the back fold under and get quilted in with the top, if that makes sense to you. You know what I mean if it has happened to you! Ripping out quilting stitches is hard and no fun at all.

Craftsy has a free "Creative Quilt Backs" class is you're interested. Learn How To Back a Quilt In: Creative Quilt Backs I got some good pointers from this class and have used some of her ideas when piecing my backs. It's worth it just to look at Elizabeth Hartman's quilts!