According to this website (link below), coffee originally came in barrels, not bags. Anyway, 100 lb bags were used to ship bulk corn and oats for the Army's horses and mules. The bags were about 24' wide and 1 yard long and were fabricated from coarse cotton, juke sackcloth or sailcloth (is that another way of saying canvas?).
Returning to coffee, at Petersburg Major W. P. Martin instructed that coffee, sugar, beans and salt be put into bags to save wagon space and reduce the weight that a wagon had to carry.
Years of reading CW literature and that's stuff I never even knew about. Mebbe I missed that part in Hard Tack and Coffee.
Linkus