如何使字母大小在每次按下按钮时随机变化。下面是一个随机显示字母的代码，字母的颜色

public partial class Form1 : Form
{
    Color[] co = new Color[4] { Color.Red, Color.Green, Color.Blue, Color.Orange };
    int a;
    public Form1()
    {
        InitializeComponent();
    }
    Random rnd = new Random();
    char[] letters = "АБВГДЕЄЖЗИІЇЙКЛМНОПРСТУФХЦЧШЩЬЮЯ".ToArray();
    string output;

    void generate(int NoOfLetter)
    {
        output = null;
        for (int i = 0; i < NoOfLetter; i++)
        {
            output += letters[rnd.Next(0, letters.Length)];
        }
        textBox1.Text = output;
    }

    private void button1_Click(object sender, EventArgs e)
    {
        Random rndcol = new Random();
        a = rndcol.Next(0, 4);
        textBox1.ForeColor = co[a];
        generate(1);
    }
}