I can't help with the measurement, but... don't use the WS stuff my man!!
There are much better options out there. I like grout, but other people prefer real rock.
Either way, those Woodland Scenics walnut shells are for the birds!
I don't hate the WS walnut shells. If soaked thoroughly with alcohol before bonding it doesn't float. It's also not translucent, it doesn't leave residue in the rail web, and size is uniform. Ed and I agree on much, but on this point we disagree. For a well-ballasted mainline, WS ballast works well.
I do think the fine WS is still a little large for N scale though; it's about perfect for HO. However, when I built the Juniata Division, the oversized ballast looked fine with my oversized rails and oversized ties.