How do I tell the size of black, grey, and fresh tanks?

KE6IRJ wrote:
I don't have a portable water meter to tell fill volume, is there any way?

Why would you need a water meter? If you're not satisfied with the information you find online simply fill a container of known capacity with water and pour that into the tanks, multiply by how many times you have to pour from this container to fill each of the tanks, and you'll obviously then know how much each tank holds.