Bg 11.50

sanjaya uvaca
ity arjunam vasudevas tathoktva
svakam rupam darsayam asa bhuyah
asvasayam asa ca bhitam enam
bhutva punah saumya-vapur mahatma

Word for word: 
sanjayah uvaca — Sanjaya said; iti — thus; arjunam — unto Arjuna; vasudevah — Krishna; tatha — in that way; uktva — speaking; svakam — His own; rupam — form; darsayam asa — showed; bhuyah — again; asvasayam asa — encouraged; ca — also; bhitam — fearful; enam — him; bhutva — becoming; punah — again; saumya-vapuh — the beautiful form; maha-atma — the great one.

Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Sanjaya said to Dhrtarastra: The Supreme Personality of Godhead, Krishna, having spoken thus to Arjuna, displayed His real four-armed form and at last showed His two-armed form, thus encouraging the fearful Arjuna.

Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
When Krishna appeared as the son of Vasudeva and Devaki, He first of all appeared as four-armed Narayana, but when He was requested by His parents, He transformed Himself into an ordinary child in appearance. Similarly, Krishna knew that Arjuna was not interested in seeing a four-handed form, but since Arjuna asked to see this four-handed form, Krishna also showed him this form again and then showed Himself in His two-handed form. The word saumya-vapuh is very significant. Saumya-vapuh is a very beautiful form; it is known as the most beautiful form. When He was present, everyone was attracted simply by Krishna’s form, and because Krishna is the director of the universe, He just banished the fear of Arjuna, His devotee, and showed him again His beautiful form of Krishna. In the Brahma-samhita (5.38) it is stated, premanjana-cchurita-bhakti-vilocanena: only a person whose eyes are smeared with the ointment of love can see the beautiful form of Sri Krishna.