how to set height of a textbox with changing width

Set the TextMode property of the TextBox to MultiLine and adjust the width and height accordingly.

I hope this helps you well.

[EDITED]

Ok. I gave you a prototype that will help you to finish up by yourself.But there are certain factors that you should consider. Such as

1.The type of font size, font family..., used in the textbox.
2.Is it underlined ?
3.When calculating the width or height, try not to break the word, and many more factors.

Prototype.

/// <summary>
/// TextBox Resizer extension class
/// </summary>
public static class ResizeExtension
{
    /// <summary>
    /// Resize TextBox according to input value
    /// </summary>
    /// <param name="textBox">TextBox instance</param>
    /// <param name="resizeFactor">ResizeFactor value</param>
    /// <param name="maxCharacterPerLine">Max Character Per line</param>
    public static void Resize(this TextBox textBox, int resizeFactor = 8, int maxCharacterPerLine = 20)
    {
        string textValue = textBox.Text;
        if (string.IsNullOrEmpty(textValue))
            return;

        // Note: It is not completed, It just a prototype
        // ---------------------------------------------
        int width = textValue.Length * resizeFactor; // Calculate width
        int calculateHeight = textValue.Length % maxCharacterPerLine;

        // CalculateHeight should be incremental value

        int height = Convert.ToInt32(textBox.Height.Value) * (calculateHeight > 0 ? calculateHeight : 1);// Assuming that the default height > 0
        // ---------------------------------------------
        // Note: It is not completed, It just a prototype

        textBox.Width = Unit.Pixel(width); // Done
        textBox.Height = Unit.Pixel(height); // Done
    }
}

How to use it? Just call this method in your TextBox instance.E.g if your WebForm contain textbox1 instance then call the Resize method.

protected override void OnInit(EventArgs e)
    {
        base.OnInit(e);
        textBox1.Resize();
    }

I hope this helps you well.