How can I increase height of a textbox without Multiline = "true" or incrementing font size?

Try this:

textBox1.AutoSize = false;

It won't show up in the intellisense, but it will work.

To have it work with the designer, you would have to make your own TextBox:

public class TextBoxEx : TextBox {
  public TextBoxEx() {
    this.AutoSize = false;
  }
}