It depends. If you have faces and sufficient resolution to clearly detect the iris, you can use the human iris diameter, which is on average about 11–12 mm. This lets you estimate a pixel-to-real-world scale. Alternatively, you can use any other object in the scene whose real dimensions are known.
Keep in mind that circular objects are preferable. If the object is not parallel to the image plane, it will appear elliptical; in that case, you should use the largest visible diameter (the major axis of the ellipse), which best approximates the true diameter.