It is not clear exactly to which region Joshua is referring. It may be somewhere to the east of the Jordan (2 Samuel 18:6 lists the forests of Ephraim as being east of the Jordan). The Rephaites were associated primarily with Bashan in the east, though the Perizzites were somewhere in the west.1
15 And Joshua said to them, “If you are a numerous people, go up by yourselves to the forest, and there clear ground for yourselves in the land of the Perizzites and the Rephaim, since the hill country of Ephraim is too narrow for you.”