Generally, target feature sizes are straightforward trigonometric calculations. For repeated patterns such as 4-bar targets, make sure that you distinguish between “bar width” and “cycle width”, a cycle being a bar and the adjacent space. Apart from that distinction, the calculation of feature sizes uses the same formula for any shape: bars, squares, round apertures, etc.
To calculate the width of a target feature:
feature size in inches = .001 x collimator focal length in inches x feature size in milliradians
feature size in inches = .0005 x collimator focal length in inches / cycles per milliradian
The above formulas are approximations, and work well for feature sizes less than 100 milliradians.
The bars in a 4-bar pattern typically have a 7:1 aspect ratio with equal bar and space widths, so the resulting pattern (4 bars and 3 spaces) is square.