The short answer: just enough to bury his dead body. Though a firm supporter of emancipation, Tolstoy was concerned that many of the newly freed serfs would lose their quasi-mystical connection with the soil. Instead of seeing it as a place to call home, as the location of a thriving community, they would, feared Tolstoy, look at the land beneath their feet as an object, a commodity to be bought and sold.

That's precisely what happens in the case of Pahom. He wrongly thinks that he needs as much land as he can get his hands on in order to lead a happy life. But his insatiable greed leads to his untimely death, far from his ancestral home. Tolstoy presents Pahom's death as a salutary warning against the dangers of rootlessness as well as greed. However, once Pahom's corpse is buried, he will symbolically reestablish the connection with the soil that he lost through his exploitation of land as an economic commodity.

The answer to your question and the title is revealed in the last part of the story when the servant digs a grave long enough to bury Pahom. The grave, which was six feet long, stretched from his head to his heels, and it turned out to be all the land he needed.

Pahom believed all his troubles would be over with enough land. He claimed that he would not fear the devil if he had enough land. Unfortunately, the devil heard Pahom’s statement and set out to destroy him. The devil made it possible for Pahom to acquire some land, but Pahom faced administrative challenges and opted to relocate. In his quest for better conditions, Pahom became greedy and wanted more land. The devil took advantage of Pahom’s weakness and led him to his grave.