FemtoDuino is probably the smallest 328 based Arduino compatible board around. Those things are tiny: foot print is 20.7×15.2mm. They’re an Atmega328 that is arduino compatible. Hands on with the super tiny arudino: FemtoDuino article and this video give you an introduction to those interesting looking small boards.
The downside of them is that they not directly breadboard friendly (hole spacing is 0.05 inch), but if you are planning to integrate them inside small gadgets. Everything needed to make your own FemtoDuino is provided on the website: schematics, kicad files, bill of materials,