I don't suppose the size of the rings really matter, does it? I don't know that there's an official rule. Just make them coordinate with the age and skill level of who's playing - bigger rings for smaller kids, smaller rings for better players. I think I would use hoola hoops just to make things simple. Throw some hoola hoops on the ground, back up several paces and make a start line for throwing.
Anyone else have other thoughts?
The lawn darts game in question as well as other outdoor games and suggestions can be found here.