Matsyapurana 93.124

93.124

vitastimātrā yoniḥ syāt ṣaṭsaptāṅgulavistṛtā
kūrmapṛṣṭhonnatā madhye pārśvayoścāṅgulocchritā //

The receptacle should be one vitasti (half a cubit, or 12 fingers) in extent, but the height should be of 6 or 7 fingers. It should be raised in the centre as the back of a tortoise, and one finger high on the sides.—124.