What is the largest non-polymeric heterostructured molecule possible? I must admit I'm not a chemist but a biologist, so please forgive me if I'm getting some of my terminology wrong. I want to ask a question about the biggest possible complex molecule that can exist in Nature.

I know that certain crystals like quartz and gems like ruby and diamond can grow very large and they could be called molecules. Also, polymers can grow very long with a complex 3D structure.

However my question is about a chemical compound which isn't a polymer but has a complex non-repetitive 3D structure of many covalently bound atoms. If we rely only on synthesis and try to construct the biggest molecule possible how big can we get?

Does the answer vary if we are talking about molecular size, molecular mass, number of atoms, 3D structure (I imagine there can be some very big molecules containing cavities which are lighter and have fewer atoms than other who have more atoms and are heavier but are dense) and so on ?

Or is there a single class or maybe even a single molecular species which combines all these characteristics and can produce the largest molecules no matter what characteristic we use to measure this quantity?

If there is such a class of compounds what it is and how do we synthesize it?
We know of no inherent limitation concerning the size of a molecule. As you pointed out, certain macroscopic entities like diamonds, graphite or charcoal can be considered fully macroscopic molecules to a certain extent (especially diamond). If chemists let their imagination run loose, they can draw whatever size a molecule you wish to create and with the desired complexity.

Whether it can be synthesised is another matter. Even very small molecules, especially natural products, can be very resistant against attempts to synthesise them. The larger a molecule gets, the more mass a rather small sample will have. It will get increasingly tedious but by no means impossible. However, unless you have oil money, Silicon Valley money or a billionaire funding you, you will likely not get the chance to finish what you want.

One of the most well-known large and not inherently repeating (or so Nicolaou says) molecules is maitotoxin whose total synthesis is famously still being attempted by the Nicolaou group.