[Justin m]

What the you have to do is determine the width of the target in pixels, given a constant value w/ the target centered, and find the hypotenuse, which is your distance
EDIT: Use the height for distance, not the width.The width can vary with angle to the target.
Example:
Note, these are necessary constants

    public static final int IMAGE_WIDTH = 320;
    public static final int IMAGE_HEIGHT = 240;
    public static final double TARGET_WIDTH = 2.0;
    public static final double TARGET_HEIGHT = 1.5;
    public static final double RECTANGLE_QUALITY_THRESHOLD = 95.0;
    public static final double PARTICLE_TO_IMAGE_THRESHOLD = 0.05;
    public static final double PARTICLE_AREA_THRESHOLD = 250.0;
    protected double fieldOfViewWidth = 0.0;

    /**
     * Return distance to target in inches
     *
     * @param targetHeightInPixels
     * @return
     */
    public double GetDistanceToTarget(int targetHeightInPixels)
    {
        double fieldOfViewHeight = TARGET_HEIGHT / targetHeightInPixels * IMAGE_HEIGHT;
        distance = (fieldOfViewHeight / 2.0) / 
                /*
                 * tan 23.5 degrees
                 */
                  0.4348123749;
        return distance * 12.0 * 1.4;
    }

I am assuming you can figure out how to filter for targets.