If you are talking about a physical long arm:
* Bras long: This is the most literal translation.
* Grand bras: Meaning "big arm," this can also be used to describe a long arm.
If you are talking about a metaphorical "long arm" (meaning power or influence that reaches far):
* Bras long: This can be used metaphorically, but it's less common than other options.
* Mainmise: This word means "control" or "domination" and implies a strong influence.
* Influence étendue: This literally means "extended influence," which is more explicit than "long arm."
* Pouvoir étendu: This means "extended power," and is another way to describe a long-reaching influence.
If you are talking about a specific legal term:
* Bras long de la justice: This is a specific phrase meaning "the long arm of the law."
It's important to consider the context of your sentence to choose the best translation.