Considerations to be made are:

Size of Pillar: Is the width of the pillar in proportion to the height?
the pillar should not go smaller than 350 ml to accommodate the stone.
Shape: Square or tapered, consider the gradient against the finished height, normally the width would reduce approximately 200ml on a standard height of 2.4. Your taper would run from 600ml at the base to 400ml at the top.
Half Height: How will your pillar be capped - either plaster, stone cills or flush ?
Down pipes: Can these be hidden within the pillars or columns ?