How many letters does the top answer for “a sign posted in a public place as an advertisement” have?

The most common answer is placard (7 letters).

Crossword clue: a sign posted in a public place as an advertisement

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

placard 7 letters wn-def

a sign posted in a public place as an advertisement

poster 6 letters wn-def

a sign posted in a public place as an advertisement