Sage Narada describes Sri Raama to be having knee-length arms, in Baala Kanda.
महोरस्को महेष्वासो गूढजत्रुररिन्दमः |
आजानुबाहुः सुशिराः सुललाटः सुविक्रमः || १-१-१०
"He is lion-chested, with concealed collarbones, knee-length are his arms, and his is a long bow, an enemy-subjugator, and his emperor's countenance is with a crowning-head and an ample forehead, and his gait is majestic like that of a lion..
However, this concept of knee-length arms was not continued in subsequent Sargas of Ramayana.
Be it Sri Raama or Ravana or Dasaratha, or someone else, Sage Valmiki describes him to be as having strong/long arms - महाबाहु or दीर्घ बाहू
विपुल अंसो महाबाहुः कम्बु ग्रीवः शुभ आननः |
गूढ जत्रुः सुताम्र अक्षो रामो देवि जनैः श्रुतः || ५-३५-१५
"O princess! Rama is a broad shouldered and a long-armed man. He has a shell-like neck. He has a handsome countenance. He has a hidden collar-bone. He has beautiful red eyes. His fame is heard about by people."
दीर्घ बाहू विशालाक्षौ शर चाप असि धारिणौ |
कस्य न स्यात् भयम् दृष्ट्वा हि एतौ सुर सुत उपमौ || ४-२-२०
"Lengthy are their arms, broad are their eyes and they are the wielders of arrows, bows and swords... and on seeing them, to whom there will be no scare? These two are like the sons of some divinity..." Sugreeva thus spoke to Hanuma.
रावणोऽयम् महाबाहुरिति संचिन्त्य वानरः |
अवप्लुतो महातेजा हनुमान्मारुतात्मजः || ५-१८-३०
The son of god of wind Vanara Hanuma of great brilliance thinking thus "This long armed one is Ravana", jumped down.
So the concept of आजानुबाहुः - knee-length arms, was not that of Sage Valmiki. As Sargas 1 to 4 of Bala Kanda are interpolations, the concept of आजानुबाहुः - knee-length arms must be a subsequent development.
However, the concept of आजानुबाहुः - knee-length arms is not without base. Very rarely , we will come across persons with आजानुबाहुः .
Some persons with Divine nature may have knee-length arms.
I had personally visited Akkalkot, Maharashtra. There I saw the blow up photo of Sri Swami Samarth. He was आजानुबाहुः - had knee-length arms.