Matsya Purana 58.16

58.16

tryaratnimātro yūpaḥ syāt‌ kṣīravṛkṣavinirmitaḥ.
yajamānapramāṇo vā saṃsthāpyo bhūtimicchatā.. 58.15 ..

The sacrificial posts should be made of the glomerous fig, measuring three cubits, with closed fist, in length, of the hand of the sacrificer. It is also auspicious to get them made of the size of the worshipper.—15.