The serving is the thread that is wrapped around the bow string to protect it. In this case it's the center serving we're talking about (where the arrow nocks on).

Some nocks are made with different groove sizes. I know at least the Easton "G" nock comes in regular and small groove but I don't recall a diameter being given with them.

Easton G-nocks are .088" for the small groove and .098" for the large groove. Small groove is best for small strings (less strands) on light draw bows. Large groove is pretty much standard for almost all adult bows.

Most standard nocks used today will measure about .095" or so. The inside of the prongs have a tit that is used to snap the nock to the string. These vary somewhat from one nock brand to another. Best thing to do if replacing the center serving is to take an arrow with the nock you plan to use and try it for fit.

Although not all nocks are made the same they should be close enough that it won't matter a lot--unless they are too tight. If you should change nocks in the future here's a trick I learned many many years ago that works fairly well, and I know I'm going to get chastized for saying it, but if the serving gets too lose due towear or nock change then all you need to do is wrap some dental floss around the serving where the arrow nocks, tie off and melt the ends. It only takes a couple wraps most of the time. Something else about dental floss is that it's slippy, almost like teflon, so the arrow leaves the string a touch cleaner. It's not what some would consider optimum, but it does work well, and you can do it yourself for free.