What is the largest non-polymeric heterostructured molecule possible?
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.