A lurker on these forums bought my 21's. He found some very sturdy 30x30x12 boxes that he sent to me. There's less than 2" of clearance in every direction with the wheels in the box.
I took apart of a bunch of smaller shipping boxes and jammed the cardboard in the sides to keep the tires firmly in the boxes. On top, I laid a flat layer of cardboard, then stuffed the remaining space with wadded newspapers. The buyer suggested NOT using styrofoam peanuts as they can scratch.
I haven't finished packing 'em all in and shipping them yet, though.