There is no ‘ideal’ distance. The more you can keep the chains from being near
vertical the better, this translates to the wider the better.
As you get wider, it gets harder to put the sled up to the same height between
them (the angle gets shallower and so more tension is needed), so adding height
to the motors may help (the shallower the angle, the more cutting error is
caused by a slight error in chain length)