You can stop your meat from drying out by searing it on all sides to seal the juices in before you put it in the oven. When you roast meat, the outside is cooked first and the juices flow into the middle of the meat if it has been seared properly. When you then take it out of the over and rest it, during the resting time, the juices flow back fairly evenly into the whole roast. This prevents it drying out so much. How long you leave it is also depends on how you like your beef. Rare, medium rare, medium or even done.
Imagine a cube with 10cm sides. Surface area is 600 sq.cm, volume is 1000cc.
Cut the cube in half, producing two 6-sided solids, 10cm x 10cm x 5cm. Volume of each is 500cc so total volume remains 1000cc. However, surface area of each solid is 4(10 x 5) + 2(10 x 10) = 400sq cm, giving a total surface area of 800sq.cm. So although the volume remains the same, the area has increased by a third. That's why it makes such a difference.