Well, it depends partly on how fast you intend to spin and the mass of the shaft, as well as the length. Crown Vic cop cars have some mighty long driveshafts for a car that can have a 3.55 rear end and a top speed of about 150mph. Because of this, they use a lighter shaft (than those used in the non-P71 cars) so the shaft won't try and shake itself to bits at top speed.