How many letters does the top answer for “in the direction of the length; in a longitudinal direction.” have?

The most common answer is lengthways (10 letters).

Crossword clue: in the direction of the length; in a longitudinal direction.

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

lengthways 10 letters web-def

running or extending in the direction of the length of a thing

lengthwise 10 letters web-def

in the direction of the length

in the direction of length. 67% similar → longitudinally

in the direction of the length 67% similar → lengthways, lengthwise, longways