Matsya Purana — Cosmography of Śākadvīpa and Successive Dvīpas: Mountains
जम्बूद्वीपस्य विस्ताराद् द्विगुणस्तस्य विस्तरः विस्तारात्त्रिगुणश्चापि परिणाहः समन्ततः //
jambūdvīpasya vistārād dviguṇastasya vistaraḥ vistārāttriguṇaścāpi pariṇāhaḥ samantataḥ //
Compared to the extent (length) of Jambūdvīpa, its breadth is twice that measure; and its circumference on all sides is said to be three times its extent.
This verse does not describe Pralaya directly; it belongs to the cosmographical section, giving proportional measurements of Jambūdvīpa as part of the ordered structure of the world.
Indirectly, it supports the Purāṇic ideal of rulership grounded in knowledge of the world-order (loka-vyavasthā): a king or householder is encouraged to understand sacred cosmology that frames pilgrimage, ritual geography, and dharmic orientation.
The verse is not a Vāstu rule, but it models proportional thinking (dviguṇa/triguṇa measures), a style of ratio-based description that also underlies Vāstu and ritual layout traditions elsewhere in the Matsya Purāṇa.