How many letters does the top answer for “the property of being strong and healthy in constitution” have?

The most common answer is lustiness (9 letters).

Crossword clue: the property of being strong and healthy in constitution

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

lustiness 9 letters wn-def

the property of being strong and healthy in constitution

robustness 10 letters wn-def

the property of being strong and healthy in constitution

the property of being strong and resolute 50% similar → stalwartness