Agniveshyagrihyasutra 3.8.1.18

3.8.1.18

tasya mātrā yadi grīvadaghnaṃ purastānnābhidaghnaṃ paścādyadi jānudaghnaṃ purastād gulphadaghnaṃ paścādyadi gulphadaghnaṃ purastāt samaṃ bhūmeḥ paścāt puruṣamātraṃ bhavatīti vijñāyate \

Its measure: if it is neck-deep in front, it is navel-deep behind; if it is knee-deep in front, it is ankle-deep behind; if it is ankle-deep in front, it is level with the ground behind; it is known [from tradition] that it is of the measure of a man.