I think I used a size 8 or 10 the last time I bought them.
Don't skimp here....Make sure you use a quality american made swivel that's not named Vision.
I've recently lost a few fish to Vision swivels where they pulled apart on fish I would have loved to have had to hand. Instead, there's a few fish swimming around on the OP with a hook and half-swivel hanging out of their mouth!
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You know something bad is going to happen when you hear..."Hey, hold my beer and watch this"