How many letters does the top answer for “relating to or formed with the infinitive” have?

The most common answer is infinitival (11 letters).

Crossword clue: relating to or formed with the infinitive

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

infinitival 11 letters wn-def

relating to or formed with the infinitive

of or relating to or formed by trees 50% similar → arboreal, arborous

of or relating to or formed with a preposition 50% similar → prepositional