The pin or the entire aperture housing? The housing max length is 2cm, the pin can be any diameter but if longer then 2cm must bend away from the sight line before hitting 2cm. (As seen in the AAE Gold aperture and the Shibuya Fiber Optic Sight Pin)
So I think the housing can be any diameter as long as is shorter than 2cm in the line of vision.
I have seen a few olympic recurve archers using several inch diameter compound apertures at the Pacific Coast Championships in CA, and the judges gave them no issue. (PCC uses WA rules, and the compound sights the OR archers used had only one fiber point, were less than 2cm long, and the bubble level was removed)
For those checking the thread but haven't read WA the rules:
"18.104.22.168. The overall length of the sighting circle or point (tunnel, tube, sighting pin or other corresponding extended component) shall not exceed 2cm in the line of vision of the athlete."
"The sight point may be a fibre optic sight pin. The total length of the fibre optic pin may exceed 2cm, provided that one end is attached outside the athlete’s line of vision at full draw, while the part within the athlete’s line of vision does not exceed 2cm in a straight line before bending. It can only provide one illuminated aiming spot at full draw. The fibre optic pin is measured independently of the tunnel."
Get yourself a matchstick and cut it exactly 2cm long.
If any part of your pin is that long, straight and points directly at your eye while you're aiming, congratulations. Your sight pin is illegal under WA rules at least.
The sight pin length is kept short so that the pin itself can't be used as an effective torque indicator.
If you had a sight pin of infinite length, (which didn't inconvenience other archers) and it was straight for 19mm, after which it bent out of your line of sight and then continued out of the galaxy and several others, it would be totally legal to use.
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