You can control the size via the Radius attribute of the particles. Unfortunately the XP SDK does not support reading the Scale value of the particles at the moment. Hopefully it will be added in a future release.
Thanks, Peter! i was a little unclear in my wording..sorry. Kann i modulate the size for example by distance to the camera?
Well, it depends on what X-Particles allows you to do. C4DtoA just reads the Radius of the particles. I'm not an expert of X-Particles, maybe someone else can assist you.
Hi Tim... you can easily modify the scale of a particle in many ways with X-Particles. If for instance, you wanted the particles to scale up as the camera approaches, you could use an xpScale set a new radius amount. In the image below, you can see that I'm using a spline to affect the scale as they approach the target.
You can then use a Sphere Falloff parented to the camera to determine how far away you want the camera to be before it begins affecting the particle scale.
You can learn more about using xpScale here http://manual.x-particles.com/modifiers/geometry-modifiers/xpscale/