The StickerExplosion class requires five fields:

ResourceLocation id;
IntList colors;
IntList fadeColors;
boolean hasTrail;
boolean hasTwinkle;

The id and colors are unmodifiable and must be set when registering. The other three parameters can optionally be set during initialization and/or by players during gameplay.

The sticker item must be registered with a corresponding StickerExplosion.

new StickerItem(new StickerExplosion(...));

The custom particle function is a consumer that provides parameters which you can use to set up your custom particles.
The scale parameter can be used scale your radius and particle sizes, and is dependent on the size of the Pokemon.
The scaleFactor parameter is determined by the scale parameter and can be used to adjust the number of particles generated based on the Pokemon size.

@FunctionalInterface
public interface CustomParticleFunction {
    void accept(ClientLevel clientLevel, double x, double y, double z, float rot, ParticleEngine particleEngine, StickerExplosion explosion, float scale, double scaleFactor);
}

Each StickerExplosion must also be registered on the Sticker Registration Event, which is run on the client side.

public class ExampleClient implements ClientModInitializer {
    @Override
    public void onInitializeClient() {
        StickerRegistryEvent.EVENT.register((event) -> {
            event.register(StickerExplosion, CustomParticleFunction);
        );
    }
}

@EventBusSubscriber(value = Dist.CLIENT, modid = Example.MOD_ID, bus = EventBusSubscriber.Bus.MOD)
public static class ClientEvents {
    @SubscribeEvent
    public static void registerStickers(StickerRegistryEvent event) {
        event.register(StickerExplosion, CustomParticleFunction);
    }
}