Every time I go to make a new quilt, I find myself Googling (again) what size that quilt should be.
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.