Astronauts on the surface of the Moon said they could block the entire view of Earth with their thumb. Using that scale, would a human being on Earth be smaller than an atom?
I’m going by an assumption that the average thumb is about one inch. Going by that, the Earth is about 5 billion thumb-widths in diameter. I’m not sure what you mean by a human being smaller than an atom, other that a human, AKA ~5-6 feet of surface area, at the size seen from the moon, compared to the size of a atom. One problem is that there is a large difference in sizes of atoms, ranging from 62 to 520 picometres. I’ll just pick carbon for simplicity’s sake, which has a diameter of 140 pm, or 140 trillionths of a meter. Mulitply 140 by 5 billion or so and we get about 700 billion pm, or .7 meters, or about 2 feet, 3 1/2 inches, which is a very small person. So yes, a very small human would be smaller than a carbon atom from the moon, or a normal human would be smaller than a much bigger atom, but the scales are right around there.
