You also asked about the size. This can be a problem since the old packing is sometimes pretty much worn down. But if one part of it can be measured, that's a start. Also measure the shaft diameter and the diameter of the threads on the seal. The packing will be just about equal to 1/2 the difference.
allows you to return packing if it has not been cut to length or inserted into the nut and compressed. Just gently try to insert the ends of the uncut packing into opposite sides of the seal when it is sitting loosely on the shaft. The correct size packing will start to slide snugly, just a little, into the gap between the nut and the shaft.