How many letters does the top answer for “wrote the periodic table” have?

The most common answer is levi (4 letters).

Crossword clue: wrote the periodic table

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

levi 4 letters wd-fact-v3

(New Testament) disciple of Jesus; traditionally considered to be the author of the first Gospel

author of the periodic table 50% similar → levi

wrote cards on the table 50% similar → christie

wrote rudolphine tables 50% similar → brahe, kepler