Fill ComboBox with Font Size

You can just fill your combobox with some size values like:

comboBox1.Items.Add("8");
comboBox1.Items.Add("9");
comboBox1.Items.Add("10");
comboBox1.Items.Add("11");
comboBox1.Items.Add("12");
...

And then you can set the font size of your textbox using something like this:

private void ComboBox1_SelectedIndexChanged(object sender, System.EventArgs e)
{
  textBox1.Font = new Font(textBox1.Font.FontFamily, Convert.ToInt32(comboBox1.SelectedItem), textBox1.Font.Style);
}