In physics, the Planck length, denoted ℓP, is a unit of length that is the distance light travels in one unit of Planck time. It is also the reduced Compton wavelength of a particle with Planck mass. It is equal to 5.72938×10⁻³⁵ m (Lorentz–Heaviside version) or 1.61623×10⁻³⁵ m (Gaussian version). It is a base unit in the system of Planck units, developed by physicist Max Planck. The Planck length can be defined from three fundamental physical constants: the speed of light in a vacuum, the Planck constant, and the gravitational constant. It is the smallest distance about which current, experimentally corroborated, models of physics can make meaningful statements. At such small distances, the conventional laws of macro-physics no longer apply, and even relativistic physics requires special treatment.
The smallest possible size for anything in the universe is the Planck Length, which is 1.6 x10 -35 m across.
It is impossible to determine the difference between two locations less than one Planck length apart. Accordingly, that is the smallest distance that ever can be measured.
Mind you, currently, we have no way of measuring anything even approaching that size. Currently, electron microscopy allows measurement of sizes as small as 50 picometers or 5 x 10 ^-11 meters. That's 10^24 larger than the Planck length, or one septillion times larger (or, one trillion trillions).