Your teacher's statement is based on the historical understanding of atoms as the smallest indivisible units of matter. However, with the progress of scientific knowledge, we now know that atoms are composed of subatomic particles, including protons, neutrons, and electrons.
Protons and neutrons are located in the nucleus of an atom, while electrons orbit around the nucleus. In terms of size, protons and neutrons are approximately the same, with a radius on the order of 10^-15 meters (femtometers). Electrons, on the other hand, are much smaller and are considered point-like particles in the current understanding of particle physics. Their size is so tiny that they are often treated as having zero size for most practical purposes.
Therefore, if we compare the sizes of protons, neutrons, and electrons, protons and neutrons have a finite size, while electrons are considered to be point-like with no spatial extent. So, in a sense, one could say that protons and neutrons are larger than electrons. However, it's important to note that the size of these subatomic particles is on a much smaller scale than the macroscopic objects we encounter in everyday life.
It's worth mentioning that our understanding of the fundamental particles and their sizes is based on experimental observations and theoretical models. As scientific knowledge advances, our understanding of the fundamental structure of matter may continue to evolve.