If a certain particle were hypothetically considered the "smallest" in the universe, it would imply that the particle is elementary and not composed of any smaller constituents. In this scenario, there wouldn't be anything smaller in terms of fundamental building blocks that make up that specific particle.
In our current understanding of particle physics, elementary particles such as quarks, leptons (e.g., electrons), and gauge bosons (e.g., photons) are considered to be point-like and without any internal structure. They are not composed of smaller particles.
However, it's important to note that our understanding of fundamental particles and their properties is based on current scientific knowledge and theories. New discoveries and advancements in physics could potentially unveil more fundamental building blocks or provide deeper insights into the nature of particles. So while it is currently accepted that certain particles are elementary and without substructure, scientific progress could challenge or refine these notions in the future.