How many letters does the top answer for “having an abnormally small head and underdeveloped brain” have?

The most common answer is microcephalic (13 letters).

Crossword clue: having an abnormally small head and underdeveloped brain

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

microcephalic 13 letters wn-def

having an abnormally small head and underdeveloped brain

microcephalous 14 letters wn-def

having an abnormally small head and underdeveloped brain

an abnormally small head and underdeveloped brain 83% similar → microcephaly

having a small brain. 50% similar → micrencephalous