Jin-Gitaxias doesn’t affect the maximum hand size of its controller.

If a player has more cards in his or her hand than his or her maximum hand size during the cleanup step (after the end step) of that player’s turn, that player discards until he or she has that many cards. A player’s maximum hand size isn’t checked at any time other than his or her own cleanup step.

Unless another spell or ability is affecting your opponent’s maximum hand size, Jin-Gitaxias’s ability will result in your opponent having a maximum hand size of zero. He or she will discard each card from his or her hand during the cleanup step at the end of his or her turn.