OK, quarks are the building blocks of protons and neutrons which make up atoms. There are 3 quarks inside a proton/neutron. A proton contains 2 up quarks and 1 down quark. Protons are really small with a diameter of 0.00000000000001m, which is hard to imagine. Protons and other types of particles that are made of 3 quarks are called baryons. We can’t see quarks on their own – they always come in 3s (or a quark and anti-quark), so we can’t measure the size of the quarks individually.
Neutrinos are smaller still! They have nearly no mass at all and therefore we don’t manage to measure the size of them at all. We say they are point like and they are too small for us to imagine.