How many letters does the top answer for “author of principia mathematica” have?

The most common answer is russell (7 letters).

Crossword clue: author of principia mathematica

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

russell 7 letters wd-fact-v3

English philosopher and mathematician who collaborated with Whitehead (1872-1970)

whitehead 9 letters wd-fact-v3

a small whitish lump in the skin due to a clogged sebaceous gland

wrote principia mathematica 50% similar → russell, whitehead