How many letters does the top answer for “what was the name of the the first episode of doctor who to air in 1963” have?

The most common answer is child (5 letters).

Crossword clue: what was the name of the the first episode of doctor who to air in 1963

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

child 5 letters otdb

a young person of either sex

unearthly 9 letters otdb

suggesting the operation of supernatural influences