Alan,
Francis T. Miller's "The Photographic History of the Civil War" … Volume VIII, page 47 has several excellent photographs of "Federal Army Wagons From the Potomac to the Mississippi."
In addition, on the same page, in describing the "omnipresent army wagons which followed the armies from Washington to the Gulf" it states:
"The dimensions of the box of these useful vehicles were as follows: Length (inside), 120 inches; width (inside) 43 inches; height, 22 inches. Such a wagon could carry a load weighing about 2536 pounds, or 1500 rations of hard bread, coffee, sugar and salt. Each wagon was drawn by a team of four horses or six mules."
Miller's 10-volume classic was published in the early 1900s and is available at many libraries.
Best regards,
Mike