What is smallest particle in an electron or a quark?
Answer today: there is no known smaller particle in an electron. And there is no known smaller particle in a quark. They are both what we call elementary particles. With current experiment technology, there have been no signs of any further substructure. More info: http://hyperphysics.phy-astr.gsu.edu/hbase/Particles/parsea.html.
That's a very, very complicated issue and depends strongly on what you mean by "size" when it comes to a subatomic particle. We can always measure the location of a particle to any arbitrary precision. This means that we can pin down its location to being within a volume of space of any arbitrary size. No matter how small this volume is, we will only ever measure one particle, never two or three or one-half. This is true for electrons as well as quarks. In this context we can say that a particle is "point-like", meaning that it has no size (though it can certainly influence an area).
We could also talk about size in the context of the size of the "probable location" we would locate the particle at during a measurement. We could say that size here means the volume of space where we have a 99% chance of locating the particle. In this context, the larger the mass of the particle, the more localized it is, meaning that the volume of space where the particle should be is smaller for particle of higher masses.
But those are simplifications. Different particles have different properties and often respond to different forces, so even this second context is problematic, as an electron works differently than a quark does. There's just no real answer.
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