“There is Mayapur and Śrī Pulina and in the middle flows the Ganges. O magnanimous one, you should understand that all of this is the abode of Gauranga. One who walks these five krosas of the dhama will see Mayapur and Sri Pulina. And whoever walks these five krosas with the devotees on the Phalguna Pūrṇimā day receives the eternal treasure of love of God.
(Sri Navadvipa-Dhama-Mahatmya, Chapter 5)

After that, every Ekādaśī My Lord went on sankirtana, wandering over the sacred islands. Sometimes He wandered over the five krosas of Antardvipa, and sometimes, according to His desire, He wandered eight krosas.
“From His house, He went first to Bārakoṇā-ghāṭa, then around Ballal-dighi, and then to Sridhara’s house. From there, He would go to the edge of Antardvipa and thus easily complete a five krosa tour. From Simuliya, He would go to the Kazi’s house. After talking to Sridhara, He would go to Gadigacha (Godruma) and then to Majidā (Madhyadvīpa). Crossing the Ganges at that point, He would go up to Pāradanga and Chinaḍāngā, on the bank of the Ganges. Then, crossing the Ganges again, He would return home, completing eight krosas.
“The complete parikramā is sixteen krosas, and if that is done, the Lord is pleased. This is the tour I am taking you on. There is no equal to this. Within this tour you will see the twelve forests of Vṛndāvana extending over sixteen krosas. This tour ends after nine nights and is thus called navaratra-parikramā in the scripture. One does the five krosa parikrama in one day, and the eight krosa parikramā is completed after three nights. The first night is spent in Mayapur, the second night in Godruma, and the third night at Pulina, the opposite bank of the Ganges.”
Hearing this instruction on the method of parikramā, Jīva Mahāśaya could not contain his love.
Desiring to attain the lotus feet of Nitai and Jahnavā, this worthless pauper describes the glories of Nadia.
(Sri Navadvipa-Dhama-Mahatmya, Chapter 10)

That person who once visits Mayapur is easily freed from the bondage of māyā. One who walks throughout Mayapur is freed from the influence of maya and the repetition of birth.
(Sri Navadvipa-Dhama-Mahatmya, Parikrama-Khanda, Chapter 3)

Srila Bhaktivinoda Thakura instructed Srila Bhaktisiddhanta Sarasvati Thakura, “Try to begin the parikramā of Śrīdhāma Navadvipa as soon as possible. Thereby everyone in the universe can receive Krsna-bhakti.” 
(Śrīla Bhaktivinoda Thakura manobhista to Sri Siddhanta Sarasvati)

The sastras say that one who walks around Navadvipa attains the fruit of millions of asvamedha‑yajnas at every step. 
(Sri Navadvipa Dham Mahatmaya, Chapter one)

One krośa equals two square miles.
(NOI 8 purport)

Information – 5 krosas ~ 10 square miles ~ 16 km approx.

Śrīla Bhaktisiddhānta Sarasvatī considered walking tours of the dhāmas beneficial both for advanced devotees, who could contemplate the Lord’s pastimes in the very locale where they had been performed, and for neophytes, who could rapidly progress by associating with such elevated Vaiṣṇavas in these most worshipable places within the entire creation. He pointed out that such parikramās afford participants an opportunity to engage simultaneously in all five of the principle practices of sādhana-bhakti: associating with devotees, chanting the holy names, hearing Śrīmad-Bhāgavatam, living in the dhāma, and worshiping the deity. By arranging to bring many devotees together in parikramā, Śrīla Bhaktisiddhānta Sarasvatī also created a new method of mass preaching, both to direct partakers and to local villagers, who would also hear from or receive prasāda from the Gauḍīya Maṭha devotees, or at least be impressed by the proceedings. And many others would learn of it from newspapers and by word of mouth.

(“Sri Bhaktisiddhanta Vaibhava” Volume Two by HH Bhakti Vikasa Swami, Chapter ‘Dhama-Seva’, Page 377)