What are you putting below the two strands of barb wire? If you are putting field fence below it (typically 4'), you are going to need more than a 6' post. For two strands above field fence, you are going to need at least 5' (or 5-1/2') of the post above ground. If that's the case, I'd say at least a 6-1/2' post, if not 7'.
However, remember it's harder to start driving tall posts.
Ken