In low mass stars the onset of Helium fusion can be very rapid, producing a burst of energy- helium flash . Eventually it settles down. If massive enough, repeat stage 5. Stellar nucleosynthesis of heavy elements. Interplay of gravity and nuclear fusion.
Electrons prevent further collapse.
Size about that of Earth. Outer layers are planetary nebula. Supernova happens when neutron star is created. Neutrons prevent further collapse. Size about that of a large city.