Have you tried tightening the compression nut slightly? Usually there should be a light weep of water while the engine is running; and a drop ~30 seconds or so when stopped. The water lubricates the flax while the engine runs.
There is a newer type of flax that uses PTFE (Teflon) as the lubricating material and this type of packing can be tightened until the packing does not drip.
As for the diameter of the packing material; you will have to remove the compression nut and remove one of the rings of flax with a dental pick type tool to check the diameter; different stuffing boxes/shafts require different diameters of packing.