Charak Samhita
नेत्रदीर्घां घृताभ्यक्तां सुकुमारामभङ्गुराम्। नेत्रवन्मूत्रनाड्यां तु पायौ चाङ्गुष्ठसम्मिताम्॥६०॥
In the mutranadi (urinary passage), it should be introduced like the bastinetra and in the payu (anal orifice) it should be of thickness upto 1 angushta.
english translation
netradIrghAM ghRtAbhyaktAM sukumArAmabhaGgurAm। netravanmUtranADyAM tu pAyau cAGguSThasammitAm॥60॥
hk transliteration by Sanscript