A macromolecule is a molecule with lots and lots of atoms. (i.e. protein, nucleic acid...)
A network solid is is a chemical compound where the atoms are "bonded by covalent bonds in a continuous network extending throughout the material."
A single grain of sand consists of, on average, ~50,000,000,000,000,000,000 atoms.
That's A LOT.
However, these atoms do bond in network covalent bonds.
But that means it's a covalent network solid.
The short answer is that sand is both a macromolecular substance as well as a covalent network solid, so I would go with it's macromolecular but you can interpret that the way you want.
