I created a new global variable:

float nextRandSize;

And then added this code at the end of setup() to generate the current random point size and the "next" size you want the point to grow to.

1.   randsize = random(40);
     nextRandSize = random(40);
     pointsize = int(randsize);
   
   

I then changed this part of draw() -- when it gets to 3 seconds, the "next" point size becomes the current point size, and I generate a new "next point size".

1.   if (millis()-hold2 > 3000) // could be (frames%24 ==0) or (frameCount%24 == 0)
     {
       sec++; 
       hold2=millis();
       println("        segundo: "+sec);
       randsize = nextRandSize;
       nextRandSize = random(40);
     }
   

I then use lerp() to set the point size when drawing. The last expression in the lerp() call is the percentage of the three seconds that has elapsed.

1. pointsize = int (lerp(randsize, nextRandSize, (millis() - hold2) / 3000));
   

I'm not sure that this is exactly what you want, but it may give you an idea.

have a look at noise() i think it might be what u need...

Oops. I didn't tell you where that last line goes.  Here's the context:

v.k. is correct, that it's much cleaner to use noise() for this, just as you are currently using it to calculate the vertical positions of the dots.  Here's a much simplified draw() loop that does it.  Change the 3 numbers at the top to control the speed of the interpolations.