This is one of those things I have to keep looking up over and over again for myself. I made my own little chart and diagram based on Quilter's Dream batting sizes since that's what my local quilting shop sells. Other brands might be slightly different sizes but this should get you in the ballpark.
You will want your quilt top to be a little bit smaller than your batting. You’d rather cut off extra batting around the edge than cut off extra quilt top around the edge.