How many letters does the top answer for “not easily perturbed or excited or upset; marked by extreme calm and composure” have?

The most common answer is imperturbable (13 letters).

Crossword clue: not easily perturbed or excited or upset; marked by extreme calm and composure

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

imperturbable 13 letters wn-def

not easily perturbed or excited or upset; marked by extreme calm and composure

unflappable 11 letters wn-def

not easily perturbed or excited or upset; marked by extreme calm and composure