I think they are M18, I'll measure one later. It might be worth checking your own element to confirm as the first link below indicates 16.05mm which may be the element rather than the tunnel and there's numerous variations to be found on german 10m rifles.
You can buy boxed sets of elements from here. I believe, but don't know, they are made by UK Neil (a bbs member).
Intershoot also do sets or singles.
I'll leave the "size issue" to those with some more experience of 10 & 20 metre disciplines.