How many letters does the top answer for “a region where a battle is being (or has been) fought” have?

The most common answer is battlefield (11 letters).

Crossword clue: a region where a battle is being (or has been) fought

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

battlefield 11 letters wn-def

a region where a battle is being (or has been) fought

battleground 12 letters wn-def

a region where a battle is being (or has been) fought