Heat the oven to 400°F with a rack in the upper-middle position. In a small bowl, stir together the cinnamon, 1 teaspoon salt and ½ teaspoon pepper. In a large bowl, toss together the pecans, 1 tablespoon oil and 1 teaspoon of the cinnamon mixture. Distribute in an even layer on a rimmed baking sheet; reserve the bowl. Toast the pecans until lightly browned, 4 to 5 minutes. Transfer to a small bowl; set aside. Reserve the baking sheet.

In the reserved bowl, toss together the sweet potato, 1 tablespoon oil and the remaining cinnamon mixture. Distribute in an even layer on the same baking sheet; reserve the bowl once again. Roast the sweet potato, stirring halfway through, until browned and tender, 20 to 25 minutes. Remove from the oven and set aside.

Meanwhile, in the reserved bowl, stir together the onion, vinegar, 2 tablespoons oil and ¼ teaspoon each salt and pepper; set aside. In a large saucepan over medium, combine the remaining 1 tablespoon oil and the couscous. Cook, stirring, until golden brown, 2 to 3 minutes. Add the cranberries, ½ teaspoon each salt and pepper and 2 cups water. Bring to a simmer over medium-high, then reduce to low. Cover and simmer, stirring once or twice, until the couscous is tender, 8 to 10 minutes. Remove from the heat, uncover and cool slightly.

To the bowl with the onion, stir in the couscous, sweet potato, arugula and half the pecans. Taste and season with salt and pepper. Serve sprinkled with the remaining pecans.