New2This -- Welcome to Pond Boss.
You've unfortunately stumbled into a deep can of worms, filled with quicksand. Plus, unfortunately, most of us here do not have a lot of experience with your type of pond. We mostly deal with earthen/clay sealed ponds, water table ponds filled to the water table, and privately owned lakes.
Please don't let this deter you. We'll be glad to answer any questions you might have.
I've been buying aggregate (river gravel, crushed gravel, stones, rocks, sand, etc.) for nearly 50 years. What a mess. There are some definitions. There are some standards, but . . . It is a murky world.
Here are two sites may help: Luck Stone Product Application Chart Crushed Stone
Anyway, I'm not sure what the "pond store" people were suggesting. Water is displaced by volume. A cubic foot of rock is a cubic foot of rock. A cubic foot of rock will displace a cubic foot of water.
Not all rocks, stones, and crushed rock/stone are created equal. Some have minerals, or other substances that can be good for fish. Others can kill your fish. I usually suggest that you put a small amount of the aggregate in a cup of vinegar. Generally, if it foams, don't use it for fish.