The smallest unit of measurement is the Planck length, which is approximately 1.616 x 10-35 meters. To put it in perspective, the Planck length is many orders of magnitude smaller than subatomic particles, making it the theoretical lower limit of length in the universe.
However, it’s important to note that in different contexts, we often use other units. For example, in terms of conventional measurements, the millimeter or the nanometer might come to mind, but they are still much larger than the Planck length.
The concept of the Planck length arises from the field of quantum mechanics and is associated with the limits of current physical theories. In general practical terms, smaller units like the nanometer (1 x 10-9 meters) are used in fields like biology and nanotechnology, but in the grand schema of physics, the Planck length reigns as the smallest measurable unit we can theorize.