mine is 6 1/4 inch. it was cut by the local kart guy to make it legal for our local track. So not sure if that’s a standard or just our tracks thing. Also I thing the longer it is the more power you get. I am semi new also so I wouldn’t take my word for gospel, its just what I have.
also fyi I am running a kt100. not sure if that pipe goes on other motors also.