The problem with that, is that if you include everything "small" in the definition, the word loses all it's meaning, feeble as it is already.
The word microplastic was introduced to describe not just any small piece of trash, but specifically that very small, invisible, pieces of plastics that are, as it turned out, everywhere, in the air, in the water, in our food, in our blood, even in space. If you add just small pieces of rubbish to it, we remove all the sense from the word, and will need another one.