We simply don't know what the smallest thing we can compute with is. A material scientist might say that the smallest silicon traces that can do the are X nanometers, then another will come along and say he is right, but we can use gallium-arsenide to get down to X-5 nanometers. And this progressive 1 upping has fueled Moore's law

There are people doing research on subatomic who think that various quarks have interesting properties for computing. This may or may not be possible. But we likely don't know yet.