• You need to serialize the private variables.
• You need to use built-in arrays or Lists.
• Private variables are not serialized (stored) by default.

Update

See this related question I just asked.

You need to explicitly tell unity the type of your array. You have to explicitly type the array with your target type. Simple. Use a built-in array or a List.

For example:

@SerializeField
private var minsize : float[];

Then you can allocate enough memory (since you can't use Add() any longer):

minsize = new float[pars.length];

Another update

You were allocating arrays in the loop. Also I cleaned up some of the code to make it a little more readable. Are you sure that you are updating the emitters somewhere along the way after you've edited them?

var pars : ParticleEmitter[];
var scale : float;

@SerializeField
private var minsize : float[];

@SerializeField
private var maxsize : float[];

@SerializeField
private var worldvelocity : Vector3[];

@SerializeField
private var localvelocity : Vector3[];

@SerializeField
private var rndvelocity : Vector3[];

@SerializeField
private var scaleBackUp : Vector3[];

function UpdateScale () {   

    var length = pars.length;

    minsize = new float[length];
    maxsize = new float[length];
    worldvelocity = new Vector3[length];
    localvelocity = new Vector3[length];
    rndvelocity = new Vector3[length];
    scaleBackUp = new Vector3[length];

    for ( i = 0 ; i < length ; i++ ) { 
        var emitter = pars[i];

        minsize[i] = emitter.minSize;
        maxsize[i] = emitter.maxSize;
        worldvelocity[i] = emitter.worldVelocity;
        localvelocity[i] = emitter.localVelocity;
        rndvelocity[i] = emitter.rndVelocity;
        scaleBackUp[i] = emitter.transform.localScale;

        emitter.minSize = minsize[i] * scale;
        emitter.maxSize = maxsize[i] * scale;
        emitter.worldVelocity = worldvelocity[i] * scale;
        emitter.localVelocity = localvelocity[i] * scale;
        emitter.rndVelocity = rndvelocity[i] * scale;
        emitter.transform.localScale = scaleBackUp[i] * scale;
    }
}

Update:

ok I am using the exact same code you posted and here is a pic of the Hierarchy:

ok sorry for answering instead of commenting but that would be the last comment, so then you could not write back.

so still not working.. I might gave overlooked something..

thanks a lot for helping me this!

here is the updated code:

var pars : ParticleEmitter[];
var scale : float;


@SerializeField
private var minsize : float[];

@SerializeField
private var maxsize : float[];

@SerializeField
private var worldvelocity : Vector3[];

@SerializeField
private var localvelocity : Vector3[];

@SerializeField
private var rndvelocity : Vector3[];

@SerializeField
private var scaleBackUp : Vector3[];


function UpdateScale () {   
    for(i=0;i<pars.length;i++){ 

        minsize = new float[pars.length];
        minsize[i] = pars[i].minSize;

        maxsize = new float[pars.length];
        maxsize[i] = pars[i].maxSize;

        worldvelocity = new Vector3[pars.length];
        worldvelocity[i] = pars[i].worldVelocity;

        localvelocity = new Vector3[pars.length];
        localvelocity[i] = pars[i].localVelocity;

        rndvelocity = new Vector3[pars.length];
        rndvelocity[i] = pars[i].rndVelocity;

        scaleBackUp = new Vector3[pars.length];
        scaleBackUp[i] = pars[i].transform.localScale;


        pars[i].minSize = minsize[i] * scale;
        pars[i].maxSize = maxsize[i] * scale;

        pars[i].worldVelocity = worldvelocity[i] * scale;
        pars[i].localVelocity = localvelocity[i] * scale;
        pars[i].rndVelocity = rndvelocity[i] * scale;

        pars[i].transform.localScale = scaleBackUp[i] * scale;
    }
}