here it is

You can do that by setting the height and width properties of the textbox before adding the controls to the page ...

Just like,

private TextBox txtBox = new TextBox();

this.txtBox.Text = "MyText";
this.txtBox.Location = new System.Drawing.Point(10, 10);
this.txtBox.Size = new System.Drawing.Size(70, 15);

this.width=150;

this.height=60;

this.Controls.Add(txtBox);