How can I convert a string length to a pixel unit?

Depends on the font, too. String length isn't sufficient.

Without using of a control or form:

using (System.Drawing.Graphics graphics = System.Drawing.Graphics.FromImage(new Bitmap(1, 1)))
{
    SizeF size = graphics.MeasureString("Hello there", new Font("Segoe UI", 11, FontStyle.Regular, GraphicsUnit.Point));
}

Or in VB.Net:

Using graphics As System.Drawing.Graphics = System.Drawing.Graphics.FromImage(New Bitmap(1, 1))
    Dim size As SizeF = graphics.MeasureString("Hello there", New Font("Segoe UI", 11, FontStyle.Regular, GraphicsUnit.Point))
End Using

Size textSize = TextRenderer.MeasureText("How long am I?", font);

In this case, I usually use a dirty, but simple way:

• I add an invisible Label that its AutoSize property is true -dirty work-.
• When I want to have the Width for a specific string, I set it to the Label.Text.
• The Width of the Label will give me the correct value.