10-15 meters

More at 1 femtometer

All particles that are built of quarks belong to one of the two families of hadrons -- baryons and mesons. Of all the hadrons, only the proton is thought to be stable. Even the neutron decays to a proton, emitting an electron and a neutrino, with a half life of just under 15 minutes. If the proton is unstable, it has a half life of at least 10³² years. (The age of the universe is roughly 10¹³ years.)

In the table below, the elementary and composite particles are grouped into two families based on the property of intrinsic angular momentum, or spin. Those particles with integer spin (0, 1, 2, ...) are called bosons, and particles with half-integer spin (1/2, 3/2, ...) are called fermions. All of the hadrons (the composite particles) are built from one of two possible combinations of the 1/2 spin quarks: Baryons are formed from three quarks and mesons are made up of one quark and one antiquark.