There is a confusion here: "molecular system" is just a figure of speech, and there is no hard line between molecular systems and e.g. macroscopic solid bodies. It is not strictly about size of bonding type. If you use the definition of molecules as entities where the atoms are connected with covalent bonds then you will end up indeed with molecules like a chromosome, or a piece of silicon single crystal.
In general, molecular systems are just a way to speak about systems which are smaller then lets say 5-10 nm, orbital picture is good to describe physical / chemical properties. The things that chemist are comfortable with. Polymers, DNA, proteins are treated generally as large molecules, because generally we look at smaller units where the molecular approach is useful. No one cares about the bangap of PE.
Solids are generally systems where the band picture is a much more useful description, and generally needs to be 100s of nanometers. 28
A piece of silicon can be discussed a a molecule, yet, people are generally much more interested on questions like band structure, dopant effect etc, where the solid state approach is much more useful.
In between: we call them nanostructures, supramolecular systems, colloids, depending on who speaks about them and how to approach them. Again, it is not necessarily about the exact size, but what kind of theory you try to apply. There are gold nanoparticles of 1-3 nm size, which is much smaller then most of the enzymes and proteins. There are even nanoparticles with fixed number of atoms, i.e. well defined stoichiometry! Yet, I heard no one calling a gold nanoparticle molecular system, just because we prefer to look at them as a small piece of metal instead of a large molecule. If you prepare a silicon particle that size, most probalbly people will categorise it as nanosystem, and not as a large molecule, in spite the fact that the atoms are connected with covalent bonds. On the other hand, we prefer to call protein complexes to supramolecular systems, because the secondary interactions between building blocks is the defining feature we care about.
Half-seriously: if you don't know if is is a molecular system, ask yourself, who would publish about this? If it is in the field of physics, than it is solid mater, maybe nano-system. If it sounds more like chemistry, then is it a molecular system, a colloid system (if it is something old and classic), or a supramolecular system. 28