Sharper shape using CC Particle World

You can use your own particles instead of usign the ones included in CC. What you need to do is to create a "custom snowflake" in the same composition using a shape layer but try not to do it too big (try a circle about 15-30 pixels). In the Particle category, change the Particle Type to Textured Square and in the Texture category you must choose the circle layer as the Texture Layer. The emitter will emit your custom particles.