The 3-fingers Grippex is the most optimal design to pull round and hexagon bars. The three fingers are automatically centring the bar.
Grippex 3-fingers bar puller comes with 2 different sizes. Grippex IIL (large) and IIS (small). The over-all dimensions depend on the fingers used. For maximum gripping force and minimum interference, use gripping fingers matching your lathe's capacity.