From the bench, 6-9" is fine. In the field, I find that 9" is not always enough to clear the grass, so I have opted for 9-13" on all of my rifles. Even that can be a bit short sometimes - I had an experience with a snow drift this past season that was less than ideal.
If the bipod is going to be used in the field, definitely go swivel. The world is not near flat enough for a fixed bipod.