How many letters does the top answer for “author of eugene onegin” have?

The most common answer is pushkin (7 letters).

Crossword clue: author of eugene onegin

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

pushkin 7 letters wd-fact-v3

Russian poet (1799-1837)

tchaikovsky 11 letters wd-fact-v3

important Russian composer whose works are noted for their expressive melodies (1840-1893)

composer of eugene onegin 50% similar → tchaikovsky

wrote eugene onegin 50% similar → pushkin, tchaikovsky