Seems most I see are 3/16" pivot barrels with 1/4" screw head sizes. There are some 1/8" with 3/16" screws also and then of course there are some guys using a 1/4" barrel with their heavy duty tactical folders. Now these are of course the ones with a straight barrel with threads all the way through and two screws. Other types are one side screw and one side smooth that recesses down in either flush and flat or somewhat recessed and dome shaped. Jantz Supply, Knifekits, and Sheffield Supply all have several examples of these in their catalogs or on line for custom makers to choose from.
In productions you can never tell what you will find and mostly I'm talking about customs for knife makers above. Productions many times like Kershaw for example, who makes their own hardware, can have some oddball size that you couldn't find if you lost it without them to send you another since they are proprietary to them made in house. Still others use some 5/32 and 7/32 barrel sizes that they must contract out to have made somewhere because they are few and far between from suppliers and many times what you are left with is finding something close but not the same exact pivot and making that work or finding one the next bigger diameter and biting the bullet to get a carbide and making the hole big enough to use the new size.
STR