How to choose size of foundry chromite sand?

Usually,the user choose the size of chromite sand in foundry field depends on the weight of the castings.

For example

large castings which is more than 50T,  choose chromite sans size AFS35-40

Miedium-size castings between 10-50T ,choose chromite sand size AFS40-45

Small castings which is less than 10T,choose chromite sand AFS45-50