I agree with merlinextral light if talking about the best crank length. It sounds like you however have multiple stokers so I think you are asking about what is acceptable rather than what is the best. I think people can adapt and use cranks that are shorter than optimal much easier than one that is a too long. I use 170mm cranks and would much prefer using 165mm rather than 175mm cranks and especially rather use 160mm than 180mm cranks.