Wow, then it is tough. As I understand it, only one of the 24 Canadian Sabers that were ordered by Israel in 55' was ever painted... and none delivered.
I can't tell the sizes, but here a couple of reference pics that may help
[url=http://idfmodelling.free.fr/sabre.html]linkname
Also, I think that you'd may be helped if you look at the other fighers used by Israel at the time (Dassault Ouragan, Mystère etc). I'm sure that IsraDecals or some other company has the proper sized roundels for you! Good luck!