I have read that the electron and other fundamental particles are considered fundamental because they have no known internal structure, i.e., there is no evidence to show that they are made up of other particles.

However, I have also read that they are thought to be of zero size. For example, the inherent size of an electron is estimated to be no more than 10^-18 m. This concept troubles me a little, since they are also described by delocalized wavepackets in quantum mechanics.

I did some reading on this here and it said:

Nevertheless, there is good reason that an elementary particle is often called a point particle. Even if an elementary particle has a delocalized wavepacket, the wavepacket is in fact a quantum superposition of quantum states wherein the particle is exactly localized. This is not true for a composite particle, which can never be represented as a superposition of exactly-localized quantum states. It is in this sense that physicists can discuss the intrinsic "size" of a particle: The size of its internal structure, not the size of its wavepacket. The "size" of an elementary particle, in this sense, is exactly zero.

My question then is, how exactly is it experimentally established that the electron is a "quantum superposition of quantum states wherein the particle is exactly localized"? And what empirical result would one expect to see if the electron were actually of inherently non-zero size?