How many letters does the top answer for “to the opposite side” have?

The most common answer is across (6 letters).

Crossword clue: to the opposite side

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

across 6 letters wn-def

to the opposite side

on or relating to the opposite side (of the body) 50% similar → contralateral

the side of an object that is opposite its front 50% similar → backside