Hi Johannes,
Yes, it is possible to change the particle size in visualization. The "Particles" tab in View Controls becomes visible after you load particles. Go under this tab and choose "Custom" for "Particles" and click "Edit Custom" to open the Custom Particle Selection dialog. "Add Selection" in the dialog, then expand the "Visualization" settings, you'll see the option of "Diameter Factor" that you can change the particle size. The more details can also be found in the section of "Custom visualization of particles" of FilterDict User Guide.