If you want to use it for weed-control/sheet mulch, especially of running grass or weeds, the bigger the pieces of cardboard the better.....there are less joints and overlaps (opportunities for runners to sneak through) And you also cover a lot more ground in a given session of mulching. I like to hit furniture and
mattress store dumpsters....all those things are shipped in HUGE cardboard boxes. Actually, usually in big plastic bags inside the boxes, so if you have uses for large pieces of plastic (frost covers?
greenhouse and coldframe covers?) there's your source for those, too.....