There really isn't one! The longest I've seen in commercially available dubbing is in packs of Fly Rite which can be a couple of inches long. The shortest staple dubbing I've ever seen used is the flue from peacock herl (I'm not joking). You want something in between. How long you make it depends on the effect you want from your dubbing, and also how fine the fibres you are mixing are. The finer the fibre the longer the length you can use.

If you want a spiky dubbing cut some up at 1/2 inch and about twice the amount shorter (1/4 to 3/8 inch). If you want a smoother effect cut the fibres longer. You need to experiment with length to get the effect you like.

Cheers,
A.