There is a wave-particle duality of all particles in nature, namely "particle" and "wave" are two forms of the same thing, or two sides of the same coin. The same object can behave like a wave in one limit, and behave like particle in another limit.

Graviton is the same as gravitational wave, more precisely graviton is the "quantum particle" of gravitational wave, just like photon is the "quantum particle" of light wave. Since gravitational wave was observed since a few years ago, in this sense we could say that graviton has been observed.

But to observe the quantum nature of the graviton, we will have to reach an extremely small length scale, which can be viewed as the size of graviton. Although a full quantum theory of gravity has not been established successfully, it is widely believed that this specific length scale will have to be as small as the Planck scale, which is 1.6×10^{-35} m. This is about 10^{20} times smaller than a proton, for example.