Need light-up ring batteries?

Hi all, I have a pewter ring with two "demon-skulls" on the sides and a screw-off skull on the front that has a battery compartment inside. The eyes are supposed to light up. Anyways, I got the ring as a gift, but the giver neither left the batteries in so I could write down the # for future reference, nor wrote it down for me. Anyways, it takes on cell, but they're smaller than AG13s (tried to fit one inside, so I know), and I have not been able to find a descending-in-order-of-size list of BCs (on cells). Please help. Thanks a lot for your time all.