Preheat oven to 425 degrees F. Line a baking sheet with foil or parchment paper and set aside. If using foil, spray with non-stick cooking spray.
Cut your potatoes into 1 1/2-inch chunks. You want them to be all about the same size (quarter the smaller potatoes, cut the larger ones into 8 pieces). Place them in a large bowl and toss with olive oil, rosemary, garlic, salt and pepper. Once the potatoes are well coated, place them on the prepared baking sheet in a single layer.
Place in oven and bake for 35-40 minutes, turning them with a spatula every 10-15 minutes, until uniformly golden and tender.