This from the 1861 Ordnance Manual under the heading "Manner of Packing Ammunition Boxes" page 279. As you can see, it does not give the dimensions of the box, just how to pack it for shipment.
For Rifled Guns. - case shot, fixed - Place upright, the balls down, resting on strips of wood as for the howitzers. The iron part of the balls rests against strips of wood 4 inches wide and .25 inch thick, nailed to the side and ends of the box at the bottom, and similar strips placed between the rows of the balls, to prevent the soft-metal cups from bearing against the box or against each other and being bruised.
..........In all the boxes, the small stores are placed in the vacant spaces on top of the ammunition. A layer of tow is placed in the bottom of each box, and the whole contents are well packed in tow, filling the box so as to be pressed down by the cover. About 3lbs of tow are required for a box.
The projectiles are referred to as "balls" , but these projectiles do have sabots referred to "soft metal cups".
Thanks
Dan