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THE FLORD FORMULA
I have been doing camera tracking work since the early days of 3D-Equalizer and every time I went on a shoot to supervise the tracking, I was stuck with the problem of choosing the right tracker size for the shot. Over the time I refined my formula and simplified it to get this one. It's tiny yet it's hugely useful since it works for every scenarios. It takes into account the different parameters that can affect the camera angle and the number of pixels in the image.
Where :
T = Tracker size (in same units as D)
P = Prefered tracker size on screen (in pixels) between 5 and 10 pixels depending on the software you will use for tracking
B = Filmback (in millimeters) see table below for reference
D = Distance from camera (in units of your choice)
F = Focal length (in millimeters)
R = Horizontal Resolution (in pixels)

So... If you are shooting a scene with a 35mm camera and a 18mm lens, transfered to NTSC, and the background is at 3 meters from the camera... you can use the folowwing formula...


...to find out that you need to put trackers that are 5 centimeters in diameter on the background to have them appear as 8 pixels big on screen.



Here is a list of the different filmbacks that are often used in production.

Format Name
 x size (mm)
 y size (mm)
35mm (Super 35) - Full Aperture 24.89
 18.67
35mm (Super 35) - 3-Perf
 24.89
 14.20
35mm (Super 35) - Transfered to HDTV
 24.89
 14.00
35mm - Academy
 21.95
 16.00
35mm - Slide
 36.00
 24.00
16mm - Standard
 10.26
 7.49
16mm - Super 16
 12.52
 7.42
70mm - IMAX
 70.39
 52.62
Video HDTV - 2/3" 16:9 sensor
 9.59
 5.39
Video HDTV - 1/3" 16:9 sensor
 5.23*
 2.94*
Video NTSC - 1/2" 4:3 sensor
 6.40
 4.80
Video NTSC - 1/3" 4:3 sensor
 4.80
 3.60
Video NTSC - 2/3" 4:3 sensor
 8.80
 6.60

* I'm not 100% sure about this one yet.