If the particle size is increased from the default, the rectangular nature of the point representation may become too apparent. Point antialiasing can be used to render the points as circles rather than squares. Benchmark the performance of antialiased points of various sizes on your system to determine the overhead of using this feature. Be sure to also take into account the fact that you will have to use alpha blending to make point antialiasing work.
If a particle must appear smaller than a single pixel, its alpha value can be reduced to make it more transparent (remember to enable blending), simulating the brightness of a smaller particle. Another technique that is faster but may not look as good is to reduce the intensity of the particle's color instead of its alpha. See Section 17.7 for more information.