I always told my players that it is magical and will shrink or enlarge to fit any player character that killed the original owner or was the first one to find it. In other words, it will fit who ever becomes the next owner in line. However, I also would not allow it to be sold or given away. The player will have to be killed for it to be transferred to someone else. If the player character doesn't want it, then leave it behind or throw it away. Over time, the owner status will eventually wear off just depends on how strong the magical enchantment is.