public class TestForm : Form 
{
   protected override void OnPaint(PaintEventArgs e)
   {
     base.OnPaint(e);
 IntPtr hDc = e.Graphics.GetHdc();
 // create a font from default Form.Font (Microsoft Sans Serif, 8.25pt), size = 8.25
 Gdi.Font font = new Gdi.Font(this.Font);
 IntPtr oldFont = SelectObject(hDc, font.Handle);
 string test1 = "1. This is drawing by GDI with GdiFont";
 TextOut(hDc, 50, 50, test1, test1.Length);
 SelectObject(hDc, oldFont);

 // here I want to a font which size is as twice as current's
 Drawing.Gdi.Font font2 = new Drawing.Gdi.Font(font, 2 * font.Size);
 oldFont = SelectObject(hDc, font2.Handle);
 string test2 = "2. This is drawing by GDI with GdiFont";
 TextOut(hDc, 50, 100, test2, test2.Length);
 font.Dispose();
 font2.Dispose();
 SelectObject(hDc, oldFont);
 e.Graphics.ReleaseHdc(hDc);

}
[DllImport("gdi32.dll", EntryPoint = "SelectObject")]
public static extern IntPtr SelectObject(
IntPtr hdc,
IntPtr hObject
);
[DllImport("gdi32.dll", EntryPoint = "TextOut")]
public static extern int TextOut(
IntPtr hdc,
int x,
int y,
string lpString,
int nCount
);
}