Using NASA’s James Webb Space Telescope, a team led by the Southwest Research Institute (SwRI) has identified a previously unknown moon orbiting Uranus,
There is no established lower limit on what is considered a “moon”. Every natural celestial body with an identified orbit around a planet of the Solar System, some as small as a kilometer across, has been considered a moon, though objects a tenth that size within Saturn’s rings, which have not been directly observed, have been called moonlets. Small asteroid moons (natural satellites of asteroids), such as Dactyl, have also been called moonlets.
Apparently there is no established size requirement