one inch tends to be the most common, the smaller sizes usualy are only found on light weight anvils.
"if you have a 1" hardy hole you can use hardies with smaller shafts, but not the other way around, so get the anvil with a 1" hardie if available
Some of the really large anvils have hardy holes 1.25 or larger but those can be troublesome because they're too big to get proper fitting hardies with much ease. a proper fit helps keep the hardy steady when using it, so while you /can/ use a hardy that's loose from it being made for a smaller hole, its' always best to use them the right size