How many letters does the top answer for “a verse line having eight syllables or a poem of octosyllabic lines” have?

The most common answer is octosyllable (12 letters).

Crossword clue: a verse line having eight syllables or a poem of octosyllabic lines

1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

octosyllable 12 letters wn-def

a verse line having eight syllables or a poem of octosyllabic lines

a verse line having ten syllables 50% similar → decasyllable