Stumled across a FREE BOARD

Hey everyone, i got a free board from a friend whos moving inland, It's a 9'3" longboard that he got from a local shaper and never even used it. It's a badass board and am gonna take it out when i can get off of work. Only thing is what size leash do i get for it, its only been ridden without one and i know to get one thats bigger than the board but i was looking for some input from all of you out there. Would really appreciate it.
I know this is a complete dumbass question. But i'm gonna be looking to teach my friends on it and dont know how long to go.
I know this is a complete dumbass question. But i'm gonna be looking to teach my friends on it and dont know how long to go.