This product ad of his is very informative:
"If you are unsure of the diameter fiber that you need, I have provided a visual comparison to US coins.
1mm fiber is approximately equal to the height of a dime,
1.5mm fiber is approximately equal to the height of a penny
and 2mm fiber is approximately equal to the height of a nickel......"
https://www.ebay.com/itm/Fluorescent...r=690022118702
For any muzzle loader front sight:
https://www.ebay.com/itm/FIBER-OPTIC...r=690755363835