If you have the old ring, measure the inside diameter.

LTG : )

If I were in your shoes I would just buy 5 or 10 of every size ring and put them in a "parts bin". Also add a few shooter tips and a few flipper bat rings to the mix and you will never have a machine down due to a bad rubber ring. All sizes up to about 4" should pretty much cover everything you would ever need. Start with mini post size and just grab 10 of each of the smaller ones and maybe 5 each on stuff over 2" and you should be good to go for a long while. It will likely cost less than dinner out would cost you to have a nice inventory of rubber rings!

You can find a "ring size" chart on Pinball Resource web site. Download & print it and you will be all set!

Use this chart.
https://user.xmission.com/~daina/tips/pub/tip0117.html

and from that ... 9 inch circumference = 2 inch rubber. done