Śatapatha Brāhmaṇa ("of a hundred paths") 10.2.2.6

10.2.2.6

taṃ vā udbāhunā puruṣeṇa mimīte puruṣo vai yajñastenedaṃ sarvam mitaṃ tasyaiṣā paramā mātrā yadudbāhustadyāsya paramā mātrā tāmasya tadāpnoti tayainaṃ tanmimīte tatropa yatprapadenābhyucrito bhavati tatpariśridbhirāpnoti tasmādu bāhyenaiva lekhām pariśridbhyaḥ khanet

He measures it (the altar) by the man with upstretched arms; for the sacrifice is a man, and by him everything here is measured; and that is his highest measure when he stands with up-stretched arms: he thus secures for him what is his highest measure, and therewith he then measures it. And what (space) there is over and above that when he is raised on the forepart of his foot, that he secures by the enclosing-stones; and hence he should dig a line for the enclosing-stones outside (the altar-ground).