As I was moving from Glasgow to Ontario this year, I lost a bridge pin (I have no idea how). I have a Yamaha FG 411S. I was wondering if any of you knew specifically what size of bridge pin this guitar uses or do any of you know how I could figure out what size i need?

I think they're all universal. If for some reason they're unique, just take one out and use it to find a match
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Bridge pins are pretty much all cheap and universal. You need to replace them now and then anyway or they bend and wear which causes bridge cracking, so just buy a new set. You can buy a set on Amazon for under $10.