Assuming a clone, although clone DTs may be cosmetically identical to an authentic, they can differ enough dimensionally to use different o-ring sizes... so your particular Stingray clone may not be representative of other clones, or the authentic.
If you have a dial or digital caliper and can measure the groove depth, it would be an easy thing to recommend a size... if you don't -
Desert Vapes has a good selection, with pricing low enough for you to buy all sizes offered.