Moses reminds the new generation how God gave the Israelites victory over their enemies—“there was not a city which we did not take from them" (verse 4), although all of them "were fortified with high walls, gates and bars" (verse 5). This, says Moses, happened because "the Lord your God has given you this land to possess" (verse 18). This reminder was to inspire confidence in the Israelites at this momentous time—encouraging them to have faith as they crossed into the Promised Land, where they would meet Canaanite resistance. Moses, rather than wallowing in self-pity over the fact that he himself would not enter the land, obeys God's command to provide this encouragement, particularly to Joshua, the new leader (verses 23-28). They need not fear the enemy since God will fight for Israel (verse 22).
Several commentaries including Tyndale and The Nelson Study Bible state that the "bedstead" of Og mentioned in verse 11 could also be translated sarcophagus. So the reference may be to the size of his coffin. These dimensions equal about 13 feet by 6 feet.