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

The most common answer is trochee (7 letters).

Crossword clue: a metrical unit with stressed-unstressed syllables

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

trochee 7 letters wn-def

a metrical unit with stressed-unstressed syllables

a metrical unit with unstressed-stressed syllables 100% similar → iamb, iambus