How many letters does the top answer for “a metrical unit with stressed-stressed syllables” have?

The most common answer is spondee (7 letters).

Crossword clue: a metrical unit with stressed-stressed syllables

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

spondee 7 letters wn-def

a metrical unit with stressed-stressed syllables

a metrical unit with unstressed-stressed syllables 80% similar → iamb, iambus