The label on the batting will usually tell you percentage of shrinkage (from the original size) expected from washing. Add a few inches more to that for the quilting and you can figure this out fairly easily.
E.g. a 100" wide quilt will wash-shrink about 5% or 5 inches for many cotton battings, leaving you with approx 95" quilts.
If the quilting takes up another 1-3", then your quilt would finish about 91-93" over-all.
You would plan to add extra border inches to get it back to the size you actually want it to finish.
Jan in VA