I know Krishna showed his Vishwaroopa Darshna during Mahabharatha War. But Shiva Puranam by DSR Anjaneyulu, which was translated to English from Telugu by RM Chella says, Rama had shown Vishwaroopa Darshan to Sati Devi.
As Sati bowed to Rama, he said: “O Mother, what made you, the wife of Siva, assume the form of my wife? What is the reason behind you leaving your husband and roaming in this wild jungle? Don’t you know that this place is full of demons? Please be welladvised and go back to Mount Kailasa”. Sati Devi was amazed. She regretted her ruse, and fell prostrate at Rama’s feet, begging his pardon. Then Lakshmana also realised the truth and the might of his divine brother.
Then Lord Rama displayed his universal manifestation to Sati Devi. Sati was struck with reverential awe, and sang the Lord’s praises: “Lord, I am able to see the entire world of creation in Thee. There! I can see the sun and the moon; the Lord Creator Brahma, the Lord Siva Himself; the thirty million gods in you. In a trillion forms of creatures, with their countless arms and faces and feet, art Thou now mirrored. Thou art indeed the Alpha and the Omega of all creation, and yet Thou hast be beginning or middle or end. I discern the infinitely iridiscent transfiguration of Thine spreading itself in the multiplicity of the Rudras, Aadityas, Vasus, Saadhyas, Asvins, Maruts, Gandharvas, Yakshas and the Titans--- all of whom are now gazing at Thine awe-inspiring Light of lights. Oh Lord of grace and mercy, I Knew not Thine absolute power in my former ignorance; now I realise Thine undening glory; so on my bended knees I beseech Thy forgiveness”.
Lord Siva told Sati: “You have been very fortunate to be able to see that great god’s Vishvarupa. You were indeed for ever sanctified by being addressed as ‘Mother by that supreme being. Now, since Vishnu, Brahma and myself are brotherly facets of one and the same Invisible Existence reflected in the Trinity, virtually you are my brother, too!”.
But Vishwaroopa Darshan wasn't mentioned in this unabridged Shiva Purana.
Did Rama show Vishwaroopa Darshan to Sati Devi?