I wouldn't reuse unless it was your only option.
I do not remember what size they are, just that when I blew 2 of them out all I had to do was take them in to a local auto parts store (NAPA) and they quickly matched it to a common core-plug from Dorman.
If you have a reasonably non-deformed old plug in hand, a decent parts store should be able to help. If they are not competent enough, ask them to let you look through their drawers of Dorman parts (or equivalent supplier); match the outside diameter and you should be good.
IIRC, you should coat the edge with Loctite (red???). I could be wrong on this, so research a little.