Some interesting facts gleaned from Wikipedia:
The diameter of a US half dollar coin is 1.205" (30.61 mm) and the diameter of a US nickel is .807" (21.21 mm)
The average diameter of a male areola is 1.1" (28 mm) and the average diameter of a "sexually mature" woman is 1.5" (38.1 mm) but can be 4" (100 mm) or more. Also noted, "Human areolae are mostly circular in shape, but many women and some men have areolae that are noticeably elliptical."
Concerning the nipple itself, "The average projection and size of human female nipples is slightly more than 3/8 of an inch (10mm)." I didn't see anything on the average male nipple.
Before I started HT, I measured my areola and recorded 1.25" (31.75 mm). I noticed that the Montgomery's glands (the small bumps on the areola that look like goose bumps) were usually somewhat contracted. I'd guess there's probably 25 - 30 visible. Since starting HT, I often see my areola are much smoother, less bumps, and that allows them to be a little larger in diameter, about 1.5" (38.1 mm). It's funny, if I look at them in the mirror for a little bit, the Montgomery's glands start contracting and the areola shrink some because of it. lol
All the best,
--AshleyP