how can i make my realitykit particles swell in size initially and then get smaller again

To make your RealityKit particles swell in size initially and then get smaller again, you can use a combination of scaling and animation techniques. While the specific context from WWDC doesn't directly address particle scaling, you can draw inspiration from the sessions on creating custom visual effects and animations.

Custom Visual Effects with SwiftUI: You can create a shader function that manipulates the scale of your particles over time. This is similar to how ripple effects are created using shaders in SwiftUI. You would define a shader that adjusts the scale based on time or another parameter, and then apply this shader to your particles.
Animation Techniques: Use RealityKit's animation capabilities to animate the scale of your particles. You can define a keyframe animation that increases the scale to a maximum value and then decreases it back to the original size.

For more detailed guidance, you might want to explore the session on Create custom visual effects with SwiftUI (19:40), which discusses using shaders and animations to create dynamic visual effects.