Remember what has happened up to this point. Specifically, remember that Orlando has run away from the court to avoid the plans that his brother Oliver has made to kill him. He has taken his servant Adam (who is quite old) with him. At the beginning of this scene, we are seeing them in the forest for the first time.
What Adam is saying here is that he is too hungry to walk any farther. He says he is going to die. That is the meaning of "I die for food." This phrase in modern English would be something like "I'm going to die of hunger." He says that he will measure out his grave because he is going to just lie down and die. When he is lying there, it will be clear how long his grave needs to be.
So Adam is just saying that he's too tired to go on and that he's going to lie down and die on the spot.