"sūkṣma rūp dhari siyahiṁ dikhāvā" is Chaupai 9 of the Hanuman Chalisa, which Tulsidas wrote in Awadhi. It means: Taking a tiny form, you showed yourself to Sita. Taking a fearsome form, you set Lanka on fire. Read it slowly, one half at a time, and say the Awadhi aloud.
सूक्ष्म रूप धरि सियहिं दिखावा ।बिकट रूप धरि लंक जरावा ॥sūkṣma rūp dhari siyahiṁ dikhāvābikaṭ rūp dhari laṅk jarāvā
Taking a tiny form, you showed yourself to Sita. Taking a fearsome form, you set Lanka on fire.
He enters Lanka in a form as small as a mosquito, later comes down from the tree to stand near Sita, and when his tail is set alight he grows huge yet light and leaps from roof to roof as the city burns.
Awadhi text: Goswami Tulsidas, Gita Press reading, checked against three Gita Press printings and Rambhadracharya's Mahaviri. English meaning and word glosses: Wisdom's own, following the traditional reading, edited by Ankur Shukla.