C# – How to to shrink(scale) an entire graphics structure

I’m trying to fit a lot of rectangles into a Bitmap, which will be displayed in a picturebox. In my real code, I figure out the total width and height of a rectangle that can encompass all them, and then I divide that by the size of the Bitmap, to get my scaling factor. The problem is I can’t figure out how to perform the scaling. The code below is a simple version of what I need to do.

Please keep in mind that I cannot rely on the picturebox’s scaling abilities (stretch), and I don’t want to simply apply the scale to the width and height of all the rectangles, because in my real code it wouldn’t work very well. I need a way to shrink it down in Graphics. It is important the Bitmap stays the same size that it is (300 X 300). Thanks. The below code is what I've gotten so far, but nothing changes with the size.

    using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Drawing;
using System.Windows.Forms;

namespace WindowsFormsApplication22
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        Bitmap BM = new System.Drawing.Bitmap(300, 300);
        Pen PenTest = new System.Drawing.Pen(Brushes.Red);
        private void Form1_Load(object sender, EventArgs e)
        {
             using (Graphics GR = Graphics.FromImage(BM))
            {

                GR.DrawRectangle(PenTest, new Rectangle(0,0,500,500));

                 // I need a scale of 0.60 in this example, because 300/500 = .6

                GR.ScaleTransform(.6F, .6F);//Doesn't Work. No Change at all in the size of the rectangle.


            }


            pictureBox1.Image = BM;
        }

    }
}