There is a correction here to be made. It is not SriRanganathaswamy, it is SriAnantaPadmanabhaswamy, whose idol is so large that it is not visible in one glance. You can see the Ranganatha Swamy in one glance at SriRangam in Tamil Nadu, but not the Anantapadmanabhaswamy in one glance at Thiruvanantapuram, Kerala. You have to do the darshana via three doors. At the first door, you will see the face of SriPadmanabhaswamy, Ananta Shesha, and Lord Shiva under the hand of Lord Padmanabha. In the middle door, you can see His torso along with Lord Brahma, emerging from the navel of Lord Padmanabha, and then from the last door, you can see His feet, wherein the ornaments on the feet look like an orangish hue.
[SriRanganatha Swamy of SriRangam, Tamil Nadu]
Actually, according to the temple's traditional Sthala Purana, the Ananthasayana Mahatmya, there was a Tulu Brahmin by the name Divakara Muni (in other places, the sage is known as Vilvamangalam Swamiyar) who was doing a deep penance when Lord Vishnu appeared near him in the form of a playful child. That child was very attractive, playful, and fearless. He put a condition that Swamiyar should be nice to Him and shouldn’t get angry with Him over whatever happens. Swamiyar too agreed. But then, when once that child put the saligrama which Swamiyar worshipped in His mouth, Swamiyar broke his word and scolded the child. The child disappeared. The child, while going, had said, “If you wish to see me again, you will have to come to Ananthankaadu.” Then the sage realized the child was Lord Vishnu Himself. He was extremely sad about what he did. He came to Ananthankaadu and got a vision of that child. The child entered a huge Ilappa tree. The tree fell, and a massive Padmanabhaswamy idol emerged from it.
[SriAnantapadmanabhaswamy of Thiruvanantapuram, Kerala]
This idol was so big that the Lord’s head was in Thiruvallam and the Lord’s feet were in Tripappur (also known as Trippadapuram or Thruppappoor). So, earlier that murthy was very much longer, even longer than the size of the murthy we see today. Swamiyar prayed to the Lord to shrink His size, because the Lord's form was too immense to behold in a single glance. So, it got reduced to 18 feet murthy, which can now be seen via 3 doors. According to some traditional accounts, Lord Parashurama consecrated the deity, while later traditions describe Vilvamangalam Swamiyar rediscovering the deity. Even the Bhagavata Purana tells that Lord Balarama visited this temple and had a holy bath in the Padmateertham, indicating that this kshetra was revered even before the beginning of Kali Yuga.
By contrast, the Sri Ranganathaswamy deity at Srirangapatna in Karnataka is also very large, yet devotees can still have darshana of the entire deity from a single doorway, unlike the unique three-door darshana at the Sri Ananta Padmanabhaswamy Temple.
[SriRanganatha Swamy of Srirangapatnam, Karnataka]
Hope this helps!
Jay SriRam!
ShriKrishnarpanamastu!