First you know you have to have the holes redrilled so the inserts and slug will fit. So if you do that, you might as well just let the pro size you while there. That being said.
Most insert's require a 31/32" hole to fit in. then you pick the one with your finger size.
As for the thumb slug, it depends on the size of your thumb what size you would get. You don't want one too small or the walls of the slug would be too thin once they drill the hole.
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