Day Six's original arrows (HD models) are 4mm/.166". They just recently introduced a 5mm/.204" line named XD.I shoot the Day Six arrows. They are skinnier than the Easton axis. Not sure on the diameter, but small.
A thinner shaft can be easier to tune in my experience, but not required. I like the Day Six because they have the thickest walls of any arrow I've seen and are very durable. As trad archers we sometimes miss and durability is important to me.