how to set random text size on each letter once? (processing)

how can I set a random size for each letter that spreads out? I want each of them to have a different random size but I can't really figure out where to put textSize (random(10, 20)); because if I put it under draw it just loops over and over again that each frame the text size of each letter changes but when I put it under setup it just changes the size of all the letters.

String message = "Alice fell into the rabbit hole";
int x =  130;
int y = 90;

Letter[] letters;

void setup() {
  size(260, 200);
  letters = new Letter[message.length()];
  textSize (random(10, 20));

  for (int i = 0; i < message.length(); i++) { 
    letters[i] = new Letter(x, y, message.charAt(i));
    //x += textWidth(message.charAt(i));
  }
}

void draw() { 
  background(255);
  for (int i = 0; i < letters.length; i++) { 
    letters[i].display(); 
    letters[i].shake();
  }
}

class Letter {
  char letter;
  float x, y;

  Letter (float x, float y, char letter) {
    this.x = x;
    this.y = y;
    this.letter = letter;
  }

  void display() {
    fill(0);
    textAlign(CENTER, CENTER);
    text(letter, x, y);
  }

  void shake() {
    x += random(-2, 2);
    y += random(-2, 2);
  }
}