We explore the possible interiors of Proxima Centauri b, assuming that the planet belongs to the class of dense solid planets (rocky with possible addition of water) and derive the corresponding radii. To do so, we use an internal structure model that computes the radius of the planet along with the locations of the different layers of materials, assuming that its mass and bulk composition are known. Lacking detailed elementary abundances of the host star to constrain the planet's composition, we base our model on solar system values. We restrained the simulations to the case of solid planets without massive atmospheres. With these assumptions, the possible radius of Proxima Centauri b spans the 0.94-1.40 R⊕ range. The minimum value is obtained considering a 1.10 M⊕ Mercury-like planet with a 65% core mass fraction, whereas the highest radius is reached for 1.46 M⊕ with 50% water in mass, constituting an ocean planet. Although this range of radii still allows for very different planet compositions, it helps to characterize many aspects of Proxima Centauri b, such as the formation conditions of the system or the current amount of water on the planet. This work can also help rule out future measurements of the planet's radius that would be physically incompatible with a solid planetary body.