I understand that the entrance pupil is the image of the aperture as seen from the front of the lens. However, is the size of the entrance pupil the same as the size of the effective aperture? In other words, if I measure the size of the entrance pupil by looking at it, will that be equivalent to the focal length of the lens divided by the f-number (with the focus set to infinity)? I know that sometimes this is hard to estimate because of the shape of the aperture blades, but I was wondering whether pupil and aperture can be quite different (perhaps at very low magnification).
Yes. F-number is defined in terms of focal length and entrance pupil diameter, so if you know any two of: entrance pupil diameter, f-number, and focal length, you can calculate the third number.
The equation that Wikipedia gives is:
Note this sentence from the linked article: