Mayamatam 35.51

35.51

tadgarbhārdhaṃśreṣṭhamardhārdhakaṃ taddhīnaṃ madhye'ṣṭāṃśake nandamānam
liṅgāyāmaṃ tulyanāhaṃ suvṛttaṃ kuryācchatrābhaṃ śiraḥsūtrahīnam

Half of that sanctum is the best; half of that half is the least; between them are nine measurements, with an increment of an eighth part. / The length of the linga should equal its girth, well-rounded, umbrella-shaped, and without lines on the head.