i can stuff 22 yards of prewashed fabric into the 11-7/8" x-3/8" x 13-5/8" box. the trick is to press it well, fold lengthwise (selvage to selvage) and then lengthwise again. wrap it around a 6.5" ruler. make 2 stacks of 11 yards each. smoosh down hard on the first stack as you slide it into the box. the second stack follows easily. i'll bet i could get more in if it wasn't prewashed, which seems to fluff it up quite a bit.
weight is not an issue with flat rate unless you're mailing outside the United States. the max weight of international flat rate - at least in that size box - is 20 lbs. 22 yards of fabric is well within that limit.
i can't get anwhere near as much into the other size flat rate box of the same price.
if you are a structural engineer, you can get 5 yards of unwashed fabs into a flat rate envelope but it needs to be reinforced with tape. one postal clerk fussed at me and said that constituted "modifying" the envelope. he almost refused to take it. the others i've handed packages over to didn't complain. (maybe he'd had a fight with his wife that morning. :lol: )