This has never been thoroughly addressed. I think that 300 to 1000 voxels is fine and does not change the estimated profile much. The important thing is to stay inside the brain and not grab any of the outside ring high FA values outside the brain. Depending on the input dataset, a 20x20x20 area might be too big and I'm scared that a user would use a 20x20x20 area just because that is what is in the example.