your batting and backing should be larger than your top when loaded onto the long-arm frame (4-5" larger all the way around) then after the quilting is done you lay the quilt out and trim it-square it before adding the binding.
the act of quilting draws up the fabric (much like embroidering or applique) which is why you need the backing/batting to be larger-to allow for the (drawing up) it all gets squared-cut to size before applying the binding.