The X-Particles shader reads the exported channels of an X-Particles Emitter (including the color of an X-Particles Material). If no shader is assigned to an X-Particles Emitter then the default color channel is rendered. You can also use this shader together with any other surface shader (such as connected to the Base Color channel of a standard_surface shader).
Size variation and scale of the X-Particles Material does not work with X-Particles v2.5. Only with v3.