Amazingly enough, protons and newtrons can be further broken down as they're both made up of things called 'quarks'. As far as we can tell, quarks can't be broken down into smaller components, making the the smallest things we know of.
As for our animal life, the tardigrade, or water bear, is only 0.5 millimeters long. The smallest ant species is 50% larger at 0.75 millimeters long.
