We have fabricated ultrathin Co particles with various shapes, variable thicknesses δ (2 ML<δ<22 ML), and lateral size L ranging from 100 μm to ≈100 nm. We find that all particles are magnetized in-plane at room temperature and are in a single domain state, independently of shape and size—with some remarkable exceptions. We also find that the magnetic state of a particle can be manipulated without influencing the state of the neighbors.