Change form size at runtime in C#

You cannot change the Width and Height properties of the Form as they are readonly. You can change the form's size like this:

button1_Click(object sender, EventArgs e)
{
    // This will change the Form's Width and Height, respectively.
    this.Size = new Size(420, 200);
}

If you want to manipulate the form programmatically the simplest solution is to keep a reference to it:

static Form myForm;

static void Main()
{
    myForm = new Form();
    Application.Run(myForm);
}

You can then use that to change the size (or what ever else you want to do) at run time. Though as Arrow points out you can't set the Width and Height directly but have to set the Size property.

In order to call this you will have to store a reference to your form and pass the reference to the run method. Then you can call this in an actionhandler.

public partial class Form1 : Form
{
    public void ChangeSize(int width, int height)
    {
        this.Size = new Size(width, height);
    }
}