अन्वयः

प्लवङ्गप्रवरैः the monkey troops, दृष्टः were gazing, प्लवने taking leap, कृतनिश्चयः resolved, पर्वसु during the fullmoon days, समुद्रः इव like the sea, रामवृद्ध्यर्थम् for the welfare of Rama, ववृधे began to grow in size.

M N Dutt

The foremost of the Vānaras at this stared at him, he swelled in size for the purpose of Rāma as the ocean swells on the full-moon days.

Summary

Having made up his mind to leap for the cause of Rama, Hanuman began to grow in size just as the sea grows during the fullmoon and newmoon days, while the monkey troops were watching.

पदच्छेदः

प्लवंगप्रवरैर्	प्लवंग–प्रवर (३.३)
दृष्टः	दृष्ट (√दृश् + क्त, १.१)
प्लवने	प्लवन (७.१)
कृतनिश्चयः	कृत (√कृ + क्त)–निश्चय (१.१)
ववृधे	ववृधे (√वृध् लिट् प्र.पु. एक.)
रामवृद्ध्यर्थं	राम–वृद्धि–अर्थ (२.१)
समुद्र	समुद्र (१.१)
इव	इव (अव्ययः)
पर्वसु	पर्वन् (७.३)

छन्दः

अनुष्टुप् [८]

छन्दोविश्लेषणम्

१	२	३	४	५	६	७	८
प्ल	वं	ग	प्र	व	रै	र्दृ	ष्टः
प्ल	व	ने	कृ	त	नि	श्च	यः
व	वृ	धे	रा	म	वृ	द्ध्य	र्थं
स	मु	द्र	इ	व	प	र्व	सु