This page on Amazon has the diameters:

https://www.amazon.com/gp/product/B00X2YA830/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

Although it only goes down to #7 which it lists as: SIZE: 0.98" (25 mm) height x 0.75" (19 mm) diameter

This page:

https://www.amazon.com/Ner-Mitzvah-Chanukah-Menorah-Glass/dp/B009W9A2XA/ref=sr_1_2?keywords=Ner%2BMitzvah%2BOil%2BCups%2Bsize%2B6&qid=1575469288&sr=8-2&th=1

Has the other sizes but without measurements

The Ner Mitzvah catalog also has sizes listed:

https://www.nermitzvah.com/catalog/product/view/id/3489/s/7-oil-glass/category/71/

If it is 1/2" then that would be around 12.7 mm so that's probably a 5 or 6 (I'm guessing here)