For several parameters, Shuriken is giving different ways to affect it in the editor : constant, curve, random, gradient etc. However, by script you can only affect a constant value, like for instance :

system.startSize = 5f; // Can't specify a curve or a range, it's a float !

Is there some secret ninjas ways to do that, or do I need to do every frames :

// Or curve.Evaluate(system.time/system.duration)
system.startSize = Random.Range(min, max); 

As we are talking, among other things, about Shuriken's random stuff, here is an interesting paper (unrelated to the issue here, and yet kind of, as the result above won't be similar to the builtin one, because the random value is re-sampled, thus not connected).

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[Edit] Apparently, when a float parameter is set "Random Between Two Constants", affecting a new value will set it as the second constant, and the first will then be firstValue * (lastSecondValue / newSecondValue). For instance, you set startSize in the inspector between 1 and 3. Then in Awake, you do startSize = 5; startSize is now between 1.66 and 5. It's empirical, so take it carefully. I didn't found any ways to set both values, or curves, gradient, two curves or two gradients.