The Compendium of Caraka 8.9.51

8.9.51

&jātyaśvahanavṛntena samaṃ gopucchasaṃsthitam/
raupyaṃ vā sarṣapacchidraṃ dvikarṇaṃ dvādaśāṅgulam//

equal [in thickness] to the stalk of a mālati flower or the aśvahan flower, shaped like a cow's tail,
or made of silver, with an aperture the size of a mustard seed, having two ears, and twelve finger-breadths long.