In this case they should exhibit a structure, which will show as a measurable size. At HERA very precise measurements of the size of quarks have been carried out. No measurable size could be found, and therefore, as far as we know at the moment, quarks are elementary. Their size, if they have one, must be smaller than 10-16cm, i.e. 1/1000 of the size of the proton. This is the smallest distance, which has been directly measured in physics.
Comparative size of an atom, a proton and a quark,
magnified by a factor 1015.
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