That the Building should surpass all the Churches in the World in the Beauty of its Walls, its Columns, and Marbles
It will be well, therefore, for your sagacity to make such arrangements and provision of all things needful for the work, that not only the church itself as a whole may surpass all others whatsoever in beauty, but that the details of the building may be of such a kind that the fairest structures in any city of the empire may be excelled by this. And with respect to the erection and decoration of the walls, this is to inform you that our friend Dracilianus, the deputy of the Prætorian Præfects, and the governor of the province, have received a charge from us.
For our pious directions to them are to the effect that artificers and laborers, and whatever they shall understand from your sagacity to be needful for the advancement of the work, shall immediately be furnished by their care. And as to the columns and marbles, whatever you shall judge, after actual inspection of the plan, to be especially precious and serviceable, be diligent to send information to us in writing, in order that whatever quantity or sort of materials we shall esteem from your letter to be needful, may be procured from every quarter, as required, for it is fitting that the most marvelous place in the world should be worthily decorated.
Source: Life of Constantine (New Advent)