Validating bold center link size

Code
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;

namespace Validated
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            this.btnOk.Enabled = false;
            
            this.txtBoxAddress.Tag = false;
            this.txtBoxAge.Tag = false;
            this.txtBoxName.Tag = false;

            this.txtBoxAddress.TextChanged += new System.EventHandler(this.txtBox_Change);
            this.txtBoxAge.TextChanged += new System.EventHandler(this.txtBox_Change);
            //this.txtBoxName.TextChanged += new System.EventHandler(this.txtBox_Change);

        }

        private void txtBoxAge_KeyPress(object sender, KeyPressEventArgs e)
        {
            if ((e.KeyChar < 48 || e.KeyChar > 57) && e.KeyChar != 8)
                e.Handled = true;
        }

        private void txtBox_Change(object sender, System.EventArgs e)
        {
            TextBox tx = (TextBox)sender;
            if (tx.Text.Length == 0)
            {
                tx.Tag = false;
                tx.BackColor = Color.Red;
            }
            else if (tx.Text.Length != 0)
            {
                tx.Tag = true;
                tx.BackColor = SystemColors.Window;
            }
            ValidateOk();
        }

        private void ValidateOk()
        {
            this.btnOk.Enabled = (bool)txtBoxName.Tag && (bool)txtBoxAddress.Tag && (bool)txtBoxAge.Tag; 
        }

        private void btnOk_Click(object sender, EventArgs e)
        {
            string outPut;
            outPut = "Name:" + this.txtBoxName.Text + "\r\n";
            outPut += "Age:" + this.txtBoxAge.Text + "\r\n";
            outPut += "Address:" + this.txtBoxAddress.Text + "\r\n";
            richTextBox1.Text = outPut;
        }

        private void btnHelp_Click(object sender, EventArgs e)
        {
            string outPut;
            outPut = "Name:" + this.txtBoxName.Text + "\r\n";
            outPut += "Age:" + this.txtBoxAge.Text + "\r\n";
            outPut += "Address:" +" "+ this.txtBoxAddress.Text + "\r\n";
            richTextBox1.Text = outPut;
        }

        private void txtBoxName_Validating(object sender, CancelEventArgs e)
        {
            if (txtBoxName.Text.Length == 0 || txtBoxName.Text.CompareTo("chen") != 0)
                txtBoxName.Tag = false;
            else txtBoxName.Tag = true;
            ValidateOk();
        }

        private void btnBold_Click(object sender, EventArgs e)
        {
            Font oldFont;
            Font newFont;
            oldFont = this.richTextBox1.SelectionFont;
            if (oldFont.Bold)
                newFont = new Font(oldFont, oldFont.Style & ~FontStyle.Bold);
            else newFont = new Font(oldFont, oldFont.Style | FontStyle.Bold);
            this.richTextBox1.SelectionFont = newFont;
            this.richTextBox1.Focus();
        }

        private void btnCenter_Click(object sender, EventArgs e)
        {
            if (richTextBox1.SelectionAlignment == HorizontalAlignment.Center)
                this.richTextBox1.SelectionAlignment = HorizontalAlignment.Left;
            else this.richTextBox1.SelectionAlignment = HorizontalAlignment.Center;
            this.richTextBox1.Focus();
        }
       
        private void ApplyTextSize(string textSize)
        {
            // Convert the text to a float because we'll be needing a float shortly
            float newSize = Convert.ToSingle(textSize);
            FontFamily currentFontFamily;
            Font newFont;

            // Create a new font of the same family but with the new size
            currentFontFamily = this.richTextBox1.SelectionFont.FontFamily;
            newFont = new Font(currentFontFamily, newSize);

            // Set the font of the selected text to the new font
            this.richTextBox1.SelectionFont = newFont;
        }


        private void richTextBox1_LinkClicked(object sender, LinkClickedEventArgs e)
        {
            System.Diagnostics.Process.Start(e.LinkText);
        }

        private void txtBoxSize_Validating(object sender, CancelEventArgs e)
        {
            TextBox txt = (TextBox)sender;

            ApplyTextSize(txt.Text);
            this.richTextBox1.Focus();
        }

        private void txtBoxSize_KeyPress(object sender, KeyPressEventArgs e)
        {
            // Remove all characters that are not numbers, backspace and enter
            if ((e.KeyChar < 48 || e.KeyChar > 57) &&
                                                   e.KeyChar != 8 && e.KeyChar != 13)
            {
                e.Handled = true;
            }
            else if (e.KeyChar == 13)
            {
                // Apply size if the user hits enter
                TextBox txt = (TextBox)sender;

                if (txt.Text.Length > 0)
                    ApplyTextSize(txt.Text);
                e.Handled = true;
                this.richTextBox1.Focus();
            }
        }
    }
}