The Compendium of Bheḷa 8.6.5

8.6.5

gulikāsyaṃ tu netraṃ syādanūnadvādaśāṅgulam | aṅgule karṇikāvāsaḥ tāṃ ca nakramayed gude ||

The nozzle should have a rounded tip and be no less than twelve finger-breadths [long]; / the ring should be situated at [the distance of a] finger-breadth, and one must not insert it [past that] into the rectum.