अन्वयः

कामरूपिणी who can take any form at will, सिंहिका नाम called Simhika, राक्षसी demoness, प्लवमानम् him who was speeding in the sky, तम् him, दृष्ट्वा after seeing, प्रवृद्धा expanded in size, चिन्तयामास started thinking.

M N Dutt

And it came to pass that beholding him in the act of bounding, a Rākşasi named Sinhikā, of great age, and capable of wearing forms at will, thought herself.

Summary

Seeing Hanuman speeding in the sky a demoness called Simhika who can assume any form at her will had grown out of proportion and thought on her part:

पदच्छेदः

प्लवमानं	प्लवमान (√प्लु + शानच्, २.१)
तु	तु (अव्ययः)
तं	तद् (२.१)
दृष्ट्वा	दृष्ट्वा (√दृश् + क्त्वा)
सिंहिका	सिंहिका (१.१)
नाम	नाम (अव्ययः)
राक्षसी	राक्षसी (१.१)
मनसा	मनस् (३.१)
चिन्तयामास	चिन्तयामास (√चिन्तय् प्र.पु. एक.)
प्रवृद्धा	प्रवृद्ध (√प्र-वृध् + क्त, १.१)
कामरूपिणी	कामरूपिन् (१.१)

छन्दः

अनुष्टुप् [८]

छन्दोविश्लेषणम्

१	२	३	४	५	६	७	८
प्ल	व	मा	नं	तु	तं	दृ	ष्ट्वा
सिं	हि	का	ना	म	रा	क्ष	सी
म	न	सा	चि	न्त	या	मा	स
प्र	वृ	द्धा	का	म	रू	पि	णी