Actually it's not that obvious when it comes to the ratio ParticleSize/ParticleCount in terms of performance.
The size of the particles is really performance consuming.
If you want to make smoke here's the deal :
- Particles Count / Seconds : The number of particles is not the main issue, the biggest issue you could have with it, is the fact that they are limited in the whole scene (every particles being generated in the current map as a whole). So if you have a weapon that shoots 10 bullets / seconds, with 20 000 particles generated / seconds, one guy in the map would reach the limit in one or two seconds of shooting (i don't remember precisely the limit). At that point the oldest particles would be instantly killed, disapearing with no fade out whatsoever.
- Particle Size : From what i remember, every visible particles need to be drawn by the gpu. So if i get close to a smoke, and i see 1000 particles of the size of half of my computer screen, the gpu will have to draw the equivalent of 500 * my screen resolution, which is pretty big.
- So as you see, if you have 10000 particles / seconds of a really small size, it might not give you a fps drop, but you could easily flood the particle limits, and therefore starting to have some clipping with the oldest particles.
Practical examples :
- Randomness (Size & Intensity Random) : The more you add randomness, the more you'll need generated particles in order to have something more opaque, to compensate the particles that got smaller because of the size random, and the one that got more transparent because of the intensity random. I think the Intensity random is the more critical one, because if a big particle is generated with a very low intensity, being very transparent and nearly not visible, it will still use the same performances than a very visible one.
- Intensity in itself : in that same spirit, exagerating the intensity setting make the smoke more opaque, but also less merging with the environment, especially on the edge of the sprites.
In conclusion, the most opaque smoke grenade you could do with no performance drop, would be with small/medium particle size, a good number of them per/seconds, high intensity, no size random, no intensity random...
...But it also would be the ugliest one.
As you understand it's a question of balance

i'll let you find yours.