The smallest possible size for a black hole to exist in the universe is known as the Planck black hole. It is a hypothetical black hole that is thought to be the smallest possible black hole that can exist according to the laws of physics.

The Planck black hole is characterised by its extremely small size, which is comparable to the Planck length, which is approximately 1.6 x 10^(-35) meters. This length is so small that it is beyond the reach of any current or foreseeable technology to observe or detect.

The formation of a Planck black hole is believed to be related to the fluctuations of quantum fields in the early universe. These fluctuations could lead to the creation of tiny black holes with masses on the order of the Planck mass, which is approximately 2 x 10^(-8) kilograms.

However, the existence of Planck black holes is highly speculative and has not been experimentally confirmed. There are several theoretical challenges and questions surrounding their formation, evaporation, and behaviour, which make them a subject of ongoing research in theoretical physics and cosmology.