If it's going to be folded, straight acrylic paint will buckle & shed along the fold lines. Either thin the paint about 1/8th with water so it soaks into the fibers more, or add a fabric medium to the paint (I use the GAC 900 from Golden Paints, you could also use their silk-screen medium which would give you more work time with the paint).
I'd recommend using something heavier than just cotton - "clothing weight" cotton won't hold up to that much painting at that size. You can either get banner material (available at most decent art supplies or some office supply stores), get a cheap grade of canvas fabric (also from an art supply), or look around to see if there's anywhere in your area to get recycled billboard material (that stuff is the bomb!)
15 years of working in art stores, it's like I learned something!